white paper on mediterranean climate variability and...

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White Paper on Mediterranean Climate

Variability and Predictability

Positioning MedCLIVAR as coordinated scientific activities

under CLIVAR umbrella.

To be submitted to the International CLIVAR Project Office.

Draft version 29th March 2004

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Contents

0. The Mediterranean Climate: Basic Issues and Perspectives 5

0.1 The Mediterranean and the Global Climate 8

0.2 Mediterranean Climate Internal Structure 10

0.3 Scientific Issues and Goals of the MedCLIVAR Project 13

0.4 Interactions with Other Projects and Initiatives 15

0.5 References 15

1. Past and Present Trends of Mediterranean Climate 18

1.1 Past Mediterranean Climate Variability 18

1.1.1 Introduction 19

1.1.2 Regional Mediterranean Climate Variability and Extremes

Based on Documentary Evidence 20

1.1.3 Natural Proxy-Based Reconstructions of Past Climate Over

Specific Regions of the Mediterranean 21

1.1.4 Large Scale Climate Reconstruction for the Mediterranean 22

1.1.5 Scientific Challenges for Future Research on Past Climate 24

1.2 Present Trends of Mediterranean Climate 25

1.2.1 Introduction 26

1.2.2 Observed Temperature Trends over the Mediterranean 26

1.2.3 Observed Precipitation Trends over the Mediterranean 27

1.2.4 Observed Daily Rainfall and Temperature Trends over the

Mediterranean 28

1.2.5 Possible Explanations of Trends 28

1.2.6 Scientific Challenges for Future Research 30

1.3 References 31

2a. Relations Between Climate Variability in the Mediterranean Region

and the Global Tropical Oceans: ENSO, the Indian and African Monsoons 39

2a.1 Priority Issues and Future Challenges 39

2a.2 ENSO Impact on Mediterranean (MED) Climate 40

2a.2.1 ENSO and Eastern Mediterranean (EM) Rainfall 40

2a.2.2 El Nino and Extreme Mediterranean Rainfall 41

2a.2.3 Transient and Stationary Waves Approach 41

2a.2.4 ENSO and Mid-Latitudes Relationship 42

2a.2.5 Recommended Future Research 42

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2a.3 Indian Monsoon Impacts on Mediterranean Climate 43

2a.3.1 Indian Monsoon and the Eastern Mediterranean 43


2a.4 African Monsoon Impacts on The Climate of the MED 44

2a.4.1 Background 44


2a.5 Tropical Intrusions into the MED Basin 46

2a.5.1 Background 46


2a.6 References 47

2b. Relationship Between Climate Variability in the Mediterranean Region

and Mid-Latitude Variability 51

2b.1 State-of-the-Art 51

2b.2 Some Ideas for Future Research 55

2b.3 References 56

3. Variability of the Mediterranean Sea Level and Oceanic Circulation and

their Relations to Climate Patterns 60

3.1 Climatic Forcing of the Mediterranean Sea Level and Oceanic

Variability 60

3.1.1 Background 60

3.1.2 Water Formation 61

3.1.3 Deep Water Changes 62

3.1.4 Straits 62

3.1.5 Sea Level 63

3.1.6 Models Results 64

3.2 Outlook and Future Challenges 64

3.3 References 65

4. Evaluation of the Regional Variability in Connection with Weather Patterns 69

4.1 Climatology of Cyclones in the Mediterranean 69

4.1.1 The Role of the Atlantic Storm Track and of the North

Atlantic Oscillation 69

4.1.2 Sub-Areas of Cyclogenesis, Seasonality and Generation Mechanisms 70

4.1.3 Present Trends and Future Scenarios 72

4.2 Weather Patterns and Mediterranean Environment 73

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4.2.1 Precipitation 74

4.2.2 Waves and Surges 75

4.3 Outlook: Crucial Topics for Ongoing Research 76

4.3.1 Identification of Generation Mechanisms and Relevant Processes 77

4.3.2 Connection to Large Scale Patterns 77

4.3.3 Analysis of Weather Regimes in Future Climate 77

4.3.4 Impact of Variations of Weather Regimes on Precipitation

Patterns, Surges, Waves 78

4.4 References 78

5. Role of the Mediterranean Salty Water Tongue Outflowing from Gibraltar on the North Atlantic

Deep Water Cell and on the Transitions from Glacial to Inter-Glacial Scenarios 81

5.1 Introduction 81

5.2 State of the Art 82

5.2.1 The Signature of the Mediterranean Outflow Water (MOW)

in the North Atlantic 82

5.2.2 Modelling of MOW 83

5.2.3 MOW and Climate 84

5.3 Scientific Challenges 85

5.3.1 Fundamentals 86

5.3.2 Paths of MOW 86

5.4 References 87

6. Modelling of the Mediterranean Regional Climate 90

6.1 Scientific Background 90

6.2 Current Status 91

6.3 Outlook and Future Challenges 92

6.3.1 High-Resolution Mediterranean Climate Modelling Systems 92

6.3.2 Development of Integrated Regional Modelling Systems 93

6.3.3 Multi-Model Ensemble Regional Climate Simulations 94

6.4 References 94

7. Aspects of Climate Dynamics Due to the Sun 97

7.1 References 97

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0. The Mediterranean Climate: Basic Issues and Perspectives

P.Lionello1, P. Malanotte-Rizzoli2, R. Boscolo3 and Jürg Luterbacher4

1 University of Lecce, Italy ([email protected])2 MIT, USA ([email protected])3 ICPO, UK and Spain ([email protected])4 University of Bern, Bern, Switzerland ([email protected])

The Mediterranean Region has many morphologic, geographical, historical and societal

characteristics which make its climate scientifically interesting per se (e.g. Bolle 2003). At the same

time, the connotation of “Mediterranean climate” has been extended to define the climate of other

(generally smaller) regions and hence it has its own role in the qualitative classification of the

different types of climate on Earth (e.g. Köppen, 1936). In general, the qualitative concept of

“Mediterranean” climate, is characterized by mild wet winters and warm to hot, dry summers and

may occur on the West Side of continents between about 30° and 40° latitude.

Figure 1. Sea floor data based on Smith, W. H. F., and D. T. Sandwell, Global seafloor topography

from satellite altimetry and ship depth soundings (Science, v. 277, p.1957-1962, 1997). Land

elevation data based on the GTOPO30 global digital elevation model, developed through a

collaborative effort led by staff at the U.S. Geological Survey's EROS Data Center (EDC).

The Mediterranean Sea (fig. 1), a marginal and semi-enclosed sea, is located on the western side

of a large continental area and is surrounded by Europe to the north, Africa to the south, and Asia

to the east. Its area, excluding the Black Sea, is about 2.5 million km2; its extent is about 3700 km

in longitude, 1600 km in latitude and surrounded by 21 African, Asian and European countries .

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The average depth is 1500 m. with a maximum depth of 5150 m in the Ionian Sea. The

Mediterranean Sea is an almost completely closed basin, being connected to the Atlantic Ocean

through the narrow Gibraltar strait (14.5 km wide, less than 300m deep at the seal). These

morphologic characteristics are rather unique. In fact, most of the other marginal basins have much

smaller extent and depth or they are connected through much wider openings to the open ocean.

Examples of the first type are the Black and Baltic seas, of the second the Gulf of Mexico and the

Arabian Sea, among others. The closest analogue to the Mediterranean is possibly the Japan Sea,

which, however, does not have a similar complex morphology of basins and sub-basins and is

located on the eastern side of the continental area. Moreover, high mountain ridges surrounds the

Mediterranean Sea on almost every side. Furthermore, strong albedo differences exist in south-

north directions (Bolle, 2003). These characteristics have important consequences on air masses

and atmospheric circulation at the regional scale (e.g. Xoplaki et al. 2003, 2004): the

Mediterranean sea is an important heat reservoir and source of moisture for surrounding land

areas; it represents an important source of energy and moisture for cyclone development and its

complex land topography plays a crucial role in steering air flow, so that energetic meso-scale

features are present in the atmospheric circulation; the ocean circulation is characterized by sub-

basin scale gyres defined by the geometry and topography of the basin.

Because of its latitude, the Mediterranean Sea is located in a transitional zone where mid-latitude

and tropical variability are both important and compete. Thus, the Mediterranean climate region

evolves on the north to the Marine West Coast Climate (from 40° to sub-polar regions) and on the

south to the Subtropical Desert Climate (southward of 30° or 25°). Further, the Mediterranean

climate is exposed to the Sout Asian Monsoon (SAM) in summer and the Siberian high pressure

system in winter. The southern part of the region is mostly under the influence of the descending

branch of the Hadley cell, while the Northern part is more linked to the mid-latitude variability,

characterized by the NAO and other mid latitude teleconnections patterns. However, the climate

variability patterns (teleconnections) present a large amount of synoptic to meso-scale spatial

variability, inter-seasonal and multi-decadal to centennial time variability (see sec. 0.1 and chapter

2). An important consequence is that the analysis of the Mediterranean Climate can be used to

identify changes in the intensity and extension of global scale climate pattern like NAO, ENSO and

the monsoons and their region of influence.

Another important characteristic of the Mediterranean region is the large amount of climate

information from in historical times (see section 1.1). This characteristic is unique on the global

scale and has not yet fully exploited. The continuous presence of well-organized local states and

the long tradition of scholarship and natural science produced reliable chronicles, which might

allow the reconstruction of some aspects of climate since the roman period and possibly further

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back in time. Some millennial-long climate series have already been reconstructed (e.g. the surge

of Venice, Camuffo, 1987). This availability of chronicles (see chapter 2) is complemented with

remarkably long observational records (associated with old universities and observatories of

municipalities, kingdoms and counties) mostly on the central and western-European part of the

Mediterranean region (e.g. Buffoni et al. 1999, Maugeri et al. 2002; Camuffo 2002; Barriendos et

al. 2002; Rodrigo 2002). Further, there are some regional temperature and precipitation

reconstructions based on natural proxies (e.g. Till and Guiot, 1990; Mann 2002; Touchan et al.

2003). This richness of data gives a unique opportunity for reconstruction of climate in past

historical and recent instrumentally developed times.

A further important characteristic of the Mediterranean Sea is the emergence of the first highly

populated and technologically advanced societies since, at least, 2000BC. Because of the

demographic pressure and exploitation of land for agriculture, the region presents since ancient

times important patterns of land-use change and important anthropic effects on the environment,

which are themselves interesting research topics.

Nowadays, about 400 millions people live in the countries around the Mediterranean Sea. This

densely populated area has large economic, cultural and demographic contrasts. There are

approximately 10-fold differences in GDP between the largest economies of the European Union

countries and small Middle East nations, and a 3 to 6-fold difference in the GDP per-capita

between Western European countries and the other nations. Demographic trends are also quite

different. European countries (also including non EU nations) are close to a null growth and

expected to stabilize or even decrease their population, while North African and Asian countries

are growing and are expected to double their population by mid 21st century. At difference with

European Countries, urbanization for most African Nations and is an ongoing process that is

changing the socio-economic structures of these regions. In the second half of the 20th, a 5 to 10-

fold increase in population of large towns is common for African and Asian countries, with respect

to the lower than 2-fold increase of southern European countries. Nowadays, Cairo (which

increased 4-fold since 1950) is the 20th largest town of the globe and Istanbul (which increased 8-

fold) is number 22. All these different trends are likely to produce contrasts and conflicts in a

condition of limited available resources. Moreover, different level of services, of readiness to

emergencies, technological and economical resources, are likely to result in very different

adaptation capabilities to environmental and climate changes. Poorer societies with recently

increased urbanization are likely to be critically vulnerable to weather extremes and incapable to

adapt to changing climate patterns. Hence the need is paramount for the best possible prediction

of future climate scenarios and descriptions of adaptation strategies and costs.

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The document comprises seven chapters, beside this introductory one, which overviews the

characteristics of the Mediterranean climate and summarizes the major issues more extensively

described in the following chapters. Chapter 1 describes the past and present trends of

Mediterranean Climate and the information that is available for its reconstruction. Chapter 2

describes the relations between climate variability in the Mediterranean region and the global and

mid-latitude climate “teleconnection” patterns. Chapter 3 describes the climatology of the

Mediterranean Sea circulation. Chapter 4 describes the regional characteristics of the weather

patterns. Chapter 5 describes the role of the Mediterranean salty water on the North Atlantic Deep

Water cell. Chapter 6 discusses the simulation of the regional Mediterranean climate and its

prediction in the future scenario. Finally chapter 7 briefly deals with solar activity.

0.1 The Mediterranean and the global climate

The climate of the Mediterranean region is to a large extent forced by planetary scale patterns. The

time and space behaviour, of the regional features associated with such large scale forcing is

complex. In fact, an important factor in the analysis of the teleconnections with global patterns is

the role of orography and land-sea distribution, whose complexity in the Mediterranean region

implies the presence of meso-scale structures and inter-seasonal variability in patterns that would

be otherwise much more homogeneous and persistent.

The whole picture has still to be focused and the physical processes involved need to be identified.

In spite of this, important, though incomplete, knowledge has already been established, and it is

summarized in this subsection.

However, the large-scale atmospheric circulation exerts a strong influence on the cold season

temperature and precipitation over the Mediterranean, though the strength of the relation varies

with region. The largest amount of studies on the effect of the mid-latitude variability refers to the

role of NAO (North Atlantic Oscillation), which determines a large and robust signal on winter

precipitation, which is anti-correlated with NAO over most of the western Mediterranean region

(Hurrell, 1996, Dai et al, 1997, Rodo et al 1997; Xoplaki 2002). However, in its Eastern part the

advection of moisture from the Mediterranean itself produces a more complicated situation, and

eventually other large-scale patterns, like EA (East Atlantic), play an important role (Fernandez et

al. 2003, Krichak et al., 2002), and in the central Mediterranean the Scandinavian pattern has a

strong influence (e.g. Xoplaki, 2002). This is superimposed with the effect of tropical variability,

specifically with a reduction of cyclones in the Mediterranean area during La Niña events. Tropical

variability modes like ENSO (Rodó, 2001, Mariotti et al., 2002) can be important in the parts where

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NAO influence is weaker (Rodo et al 1997). There are evidences that ENSO is significantly

correlated with winter rainfall in the Eastern Mediterranean (Yakir et al., 1996 and Price et al.,

1998). However it is still open for debate, what could be the physical mechanisms for these links.

In summer, when the advection of moisture from the Atlantic is weaker and the Hadley cell moves

northward and attenuates, there are evidences of connections with the Asian and the African

monsoons (stronger in the eastern part).

The influence of NAO on the Mediterranean temperature is weaker than on precipitation and the

observed correlation has been found to be non-linear and non-stationary (Pozo Vazquez et al

2001). Mediterranean summer temperatures have no relation with the NAO, and they are not

adequately linked to larger scale patterns. Rather, warm Mediterranean summers are connected

with blocking conditions, subsidence, stability, a warm lower troposphere and positive

Mediterranean SSTs (Xoplaki et al., 2003).

The analysis of teleconnection with global scale patterns is very important in a climate change

perspective. During the second half of the 20th century, there is a well-documented trend showing

the reduction of overall precipitation and its concentration in intense events, resulting in a

progressively drier summer season and more dangerous floods. These trends can have large

impacts on societies in the Mediterranean region. Because of the difficulty to resolve it in global

climate simulations, the identification of teleconnections with large-scale patterns is a basic tool for

the prediction of future climate conditions.

Important environmental changes have been observed in the Mediterranean Sea circulation during

the last decades. Warming trends have been observed both in deep and intermediate water (e.g.

Bethoux et al. 1990, 1998). Sea level has increased in line with the mean estimated global value

(1.8mm/year) till the 1960’s, but it has subsequently dropped by 2-3 cm. till the beginning of the

90’s (Tsimplis and Baker, 2000). During the last decade of the 20th century, sea level has

increased 10 times faster than on global scale. These trends are superimposed with a major

change that has characterized the Eastern Mediterranean with a transition in the structure of its

closed internal thermohaline cell. Historically, the Eastern Mediterranean thermohaline circulation

has been driven by a deep convection site for dense water formation localized in the southern

Adriatic (Roether et al 1983). This was in fact the situation in the 80’s. Between 1987 and 1991,

however, the “driving engine” of the thermohaline circulation shifted to the Southern

Aegean/Cretan Sea, and starting in 1991 through the late 90’s all the intermediate and deep water

of the Eastern Mediterranean was observed to spread out from the Aegean Sea through the Greek

Arch Straits (Roether et al. 1996, Malanotte Rizzoli et al 1999). This transition is known as the

Eastern Mediterranean Transient (EMT). This observational evidence has led to postulate different

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equilibria for the thermohaline circulation, which have also been found through box-models of the

basin (Ashkenazy and Stone, 2003). All these features and the circulation of the Mediterranean

Sea have been object mostly of modelling and process oriented studies, which have not addressed

the problem of its relation to global climate variability patterns. The role of local and large-scale

forcing remains presently unclear.

The Mediterranean outflow across the Gibraltar strait determines the presence of a well-known

tongue of very salty water in the entire Northern Atlantic at intermediate depths (1000 to 2500 m).

This water introduces an important signature in the salinity field and has potentially important large-

scale climate implications. Two mechanisms are presently envisioned through which the salty

Mediterranean water may precondition the convective cells of the Atlantic northern Greenland and

Labrador seas where North Atlantic Deep Water (NADW) is formed and which drive the Atlantic

Meridional Overturning Circulation (MOC). The first mechanism involves a direct advective

pathway from the Strait of Gibraltar to the polar seas, which is observed on all isopycnal surfaces

from sigma-1+ 31.938 to sigma-3+41.44 (Reid, 1994). The second mechanism involves a

progressive lateral mixing of the Mediterranean water with Atlantic intermediate water and

entrainment in the North Atlantic current that reaches the polar seas (Lozier et al., 1995). Both

mechanisms imply a significant role for the Mediterranean water in preconditioning the surface

water column of the convective cells thus increasing the volumes of newly formed NADW. The

interaction of the Mediterranean Outflow with the thermohaline circulation of the North Atlantic rises

the possibility for feedback mechanisms, eventually active both at decadal and millennial time

scales, involving the North Atlantic, the Mediterranean and the overlying atmosphere, which have

potentially important climatic implications (Rahmstorf, 1998, Artale et al 2002).

0.2 Mediterranean climate internal structure

The simulation of the Mediterranean climate is a typical problem requiring the downscaling of

global climate scenarios, that is of simulations carried out with GCM (Global Climate Models). The

resolution presently used for global climate simulations does not describe adequately the basins

that compose the Mediterranean Sea and of the mountain ridges surrounding it. The characteristic

structures of the Mediterranean region can be identified on the model land-sea mask and surface

elevation grid, only if the grid step is at least smaller than 50km. Actually, even smaller scales have

to be introduced for describing surface winds and precipitation, whose spatial variability involves

scales smaller than 10km. Therefore, the extraction of information on many environmentally

important quantities (like temperature, winds, precipitation) in the Mediterranean region, which is

characterized by a large spatial variability, requires a complicated processing of information

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provided by the global simulations, that is downscaling (e.g. Marinucci e Giorgi 1992, Giorgi et

al.1992, Dèquèt e Piedelievre 1995; González-Rouco et al. 2000; Gibelin and Déqué 2003).

Many studies, research programs and European projects have analyzed the problem of climate

simulation over Europe, including the Mediterranean in the southern part of the considered region.

The comparison between the results of long simulations carried out with LAMs (Limited Area

Models) and observations shows that relevant systematic bias are present also when the LAMs

use “perfect” lateral boundary conditions, that is boundary conditions derived from observations

(Christensen et al. 1997). Moreover, the reliability of regional climate scenarios is affected by

errors in the large-scale circulation that is used by the RCMs as lateral boundary conditions

(Machenhauer et al. 1996). In fact, with respect to global simulations, the use of RCMs produces

improvements in the reproduction of the orographic precipitation, of the structure and intensity of

the cyclones, but no clear progress on the structure of the large-scale circulation. It appears that,

because of errors in the models and in the boundary conditions, only increasing the resolution itself

cannot produce more realistic regional climate scenarios. Therefore, the results of climate change

studies are only partly significant, because the resulting climate change signal is comparable to

their systematic error, whose reduction is crucial for the evaluation of the climatic change in the

Mediterranean region. However, the various model simulations of the anthropogenic effect on

climate, tend to agree predicting in the Mediterranean region a temperature increase larger than

the global average and a modest (and controversial) precipitation decrease. Climate change

simulations produce a signal in the range from +3 to +7K for temperature and from –40% to +20%

for precipitation (Giorgi and Francisco, 2000a; 2000b).

The correct analysis of climate mechanisms internal to the Mediterranean region is not only

important to connect global scale and regional scale patterns, but also to identify internal modes.

Particularly in reference to the moisture balance, the western Mediterranean Sea represents

source for the surrounding land areas and the eastern part of the region, as the moisture released

by evaporation is redistributed by the atmospheric circulation (Fernandez et al. 2003). It would be

important to investigate whether these processes represent coupled atmosphere-ocean variability

modes and their importance for the regional hydrological cycle. Regional weather regimes are a

basic element of this variability. They are characterized by meso-scale features and several

cyclogenesis areas (e.g. Alpert et al.1990, Trigo et al. 1999). Orographic cyclogenesis is a well-

established process, which implies that many of them are triggered by synoptic systems passing

over central and Northern Europe along the Northern Hemisphere storm track. The large scale

patterns describing their variability are not well established and, though NAO plays a role, other

teleconnection patterns with centres of action located closer to or above Europe are likely to be

mostly responsible for it (Rogers 1990, 1997). The importance of regional factors (e.g.

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Mediterranean Sea surface temperature, moisture content of the air column) has also to be

assessed.

The importance of localized convection processes is determined by air-sea interactions and by the

basin thermohaline circulations. Intense cooling and evaporation over restricted areas in the north-

western Gulf of Lyon, the southern Adriatic sea and, in the 90’s, the Aegean/Cretan sea determine

the formation of dense waters filling the bottom of the basin (Western and Eastern Mediterranean

bottom waters related to the respective sources). The two sub-basins are moreover almost

disconnected at these deep/bottom levels, hence their thermohaline circulations are independently

driven by the respective sources. As previously discussed, the Eastern Mediterranean

thermohaline circulation is a closed cell endowed with multiple equilibria. Analogous observational

evidence, and related modelling studies, for the Western Mediterranean are lacking. Intense

evaporation in the Levantine basin determines the formation of LIW (Levantine Intermediate Water)

which is part of the open thermohaline cell constituted by two branches: Atlantic Water entering at

Gibraltar and making its way to the Levantine, there been transformed into LIW by intermediate

convection processes and returning all the way to Gibraltar where it finally exits forming the North

Atlantic salty water tongue. External forcing mechanisms have been invoked as responsible for the

variability of these processes. For instance, it has been shown that the wind stress climatology

over the Eastern Mediterranean was quite different in the 80’s and the 90’s. This forcing variability

may have induced the observed important change in the Eastern Mediterranean upper thermocline

circulation which undoubtedly affected the LIW pathways. Internal mechanisms may equally be at

play, such as an internal redistribution of salt in the Eastern basin (Roether et al., 1996) versus an

overall change in the surface heat budget. The construction of the Nile dam and the diversion of

Russian rivers were argued to have had an impact in the EMT event (Boscolo and Bryden, 2001)

as well as induced an increase in the overall salinity of the whole Mediterranean basin (Rohling

and Bryden, 1992) -though the competing effects of these various mechanisms is far from

understood. The Black sea outflow undoubtedly plays a role in the Northern Aegean hydrographic

conditions, even though how the Black sea induced freshening of Aegean waters may be confined

to its northern areas and no affect the dense water formation processes of the southern

Aegean/Cretan sea that led to the EMT. As a conclusion, the investigation, both from an

observational and modelling perspectives, of the different processes occurring in the different parts

of the Mediterranean, of their interactions and mutual feedbacks is far from having been explored

and even less understood.

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0.3 Scientific Issues and Goals of the MedCLIVAR project

The previous discussion suggests that the following activities are fundamental for the

understanding of the Mediterranean climate and will be promoted by the MedCLIVAR project:

• To construct homogeneous sets of data for regional climate analysis and comparison with

model simulations. Presently, often the single sets of data supply information, which is rich and

very valuable, but fragmentary and local in its scope. These homogeneous data sets should

consist of chronicle based, indirect climate reconstructions, and also include instrumental

observations, which are often disperse in the archives of many different institution, and difficult

to be accessed by researchers, with their use often being based on personal contacts. The

sets of data should be extended to establish a permanent environmental monitoring network

• To correct for the fragmentary reconstruction of the Mediterranean Sea circulation as based on

conventional oceanographic campaigns (with consequent time and space lack of resolution) via

the establishment of a permanent observational system. The key observational objectives

should include exchanges across the main straits and the identification of representative

locations recommended for continuous monitoring. Data archive shall include satellite

observations (post 1985, circa) and favour the parallel analysis of measurements campaigns,

remote sensing and “in situ” observations

• To establish an archive of model simulations relevant at the regional Mediterranean scale and

provide information that could be used for performing regional simulations. To support

exchange of information and favour coordination and exchange of data among groups of

modellers

• To provide a reference source of information to bridge the gap between research, on one side,

and authorities, public opinion and “end users’ in general, on the other side. Distribute the

information on active projects and their main results, on climate research in general via a

dedicated web-page and eventual press releases. Promote workshops and conferences on

Mediterranean climate.

The tools developed by these activities will be a fundamental support for research on the following

scientific issues and objectives, which will be addressed by the MedCLIVAR project:

• Identification of the physical mechanisms responsible for the statistically observed

teleconnections between the Mediterranean and global scale climate patterns, with focus on

the role of the meso-scale structures in the distortion of the effect of global patterns in the

Mediterranean regions. This issue is important in a climate change perspective, as

Mediterranean Sea is not adequately resolved in global climate simulation and the direct

identification of climate change signal is uncertain.

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• Understanding the link between Mediterranean weather patterns and large-scale variability

modes and assess the role of regional climatic conditions (e.g. SST patterns and mean value,

moisture content of the Mediterranean air mass) on the intensity and frequency of the

Mediterranean weather events.

• Use the Mediterranean climate variations as a proxy for changes of the relative intensity and

extension of mid-latitude and tropical climate variability patterns and of global regimes in

general.

• Reconstruction of past Mediterranean climate using multiproxy data. Analysis of the nature of

the past climate and of the causes of its variability, in order to establish if they are related to

any forcing such as solar or volcanic, or they are random internal variations (the english

structure is I think not yet correct).

• Analysis of the environmental and climatic effect of the strong anthropic influence. In the

Mediterranean region, land use change has taken place since ancient time and is related to

agriculture, forestry exploitation, urbanization, river and lakes management, and heavy

demographic pressure. The importance of this regional anthropic forcing should be compared

to that of shift and changes of global climate regimes.

• Understanding the role of solar variability and volcanism in the Mediterranean climate

combining paleo-climate model simulations and proxy based reconstructions

• Identification of the links between water formation processes (both LIW and MBW) and large

scale and Mediterranean internal climate variability modes. This involves studies of air-sea

interaction, deep convection and of the strait exchanges between sub-basins (internal to the

Mediterranean) and through the Gibraltar Strait, in order to understand the formation and the

subsequent evolution of water masses. It moreover, includes the analysis of deep and

intermediate-depth temperature of the Mediterranean Sea and of its mean sea level in order to

identify mechanisms and relations to the large-scale climatic trends.

• Assessment of the role of the Mediterranean Sea as moisture and heat reservoir for the

surrounding regions. Identification of the relevant time scales and spatial patterns.

• Evaluation of regional climate changes patterns and prediction of future scenarios. The

analysis should include the identification of adaptation goals, with focus on the hydrological

cycle, increased probability of intense floods, and decreased availability of water during the dry

season.

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0.4 Interactions with Other Projects and Initiatives

MedCLIVAR project will benefit from the cooperation with related ongoing efforts in the

Mediterranean region. The project “Eau Mediterranee en Atlantique” (EMA) coordinated by X.

Carton et al. (2004) is highly relevant to the objectives of the present effort. We have already

established contacts with EMA scientists. Another ongoing activity that MedCLIVAR will seek

linkages with is the Mediterranean Forecasting System (http://www.bo.ingv.it/mfstep/), a EU

Framework 5 project coordinated by N. Pinardi that is contributing to the GODAE.

Plans are developing for a Mediterranean GEWEX Continental Scale Experiment proposed by Y.

Tourre and J-H. Bolle. Contacts are already established with Y. Tourre for a close coordination

during the planning phase.

0.5 References

Ashenazy Y. and P. H. Stone, 2003. Box modelling of the Eastern Mediterranean, Submitted to J.

Phys. Oceanogr.

Barriendos M, Martin-Vide J, Pena JC, Rodriguez R (2002) Daily Meteorological Observations in

Cadiz – San Fernando. Analysis of the Documentary Sources and the Instrumental Data

Content (1786-1996). Clim Change 53: 151-170

Bolle, H.-J. (Ed), 2003: Mediterranean Climate – Variability and Trends. Springer Verlag, Berlin,

Heidelberg, New York, 372 pages.

Boscolo R. and H. Bryden, 2001. Causes of long-term changes in the Aegean Sea deep water.

Oceanologica Acta, 24, 519-527.

Buffoni L., M. Maugeri, T. Nanni, 1999. Precipitation in Italy from 1833 to 1996, Theor. Appl.

Climatol, 63, 33-40.

Camuffo D (2002) History of the long series of the air temperature in Padova (1725-today). Clim

Change 53: 7-76

Christensen J.H., B. Machenhauer, R. G.Jones, C. Shär, P.M. Ruti, M. Castro, G. Visconti, 1997.

Validation of present-day regional climate simulations over Europe: LAM simulations with

observed boundary conditions, Clim. Dyn., 13, 489-506

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of ensemble simulations with the HADCM2 coupled AOGCM, Clim. Dyn., 16,169-182.

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(Eds.). Handbuch der Klimatologie 3. Gebrueder Borntraeger, Berlin, 46 pages.

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18

1. Past and Present Trends of Mediterranean Climate

1.1 Past Mediterranean Climate Variability

Jürg Luterbacher1, Elena Xoplaki1, Erich Fischer1, Andreas Pauling1, Heinz Wanner1, Fidel J

Gonzalez-Rouco2, Ricardo Garcia Herrera2, Mariano Barriendos3, Fernando Rodrigo4, Joel Guiot5,

Eduardo Zorita6, Norel Rimbu7, Thomas Felis7, Jucundus Jacobeit8, Annarita Mariotti9, Michael E.

Mann10 and Ramzi Touchan11

1 University of Bern, Bern, Switzerland ([email protected], [email protected], [email protected],[email protected], [email protected])2 Universidad Complutense, Madrid, Spain ([email protected], [email protected])3 University of Barcelona, Spain ([email protected])4 University of Almeria, Spain ([email protected])5 CEREGE CNRS, Aix-en-Provence cedex 4, France ([email protected])6 Institute for Coastal Research, Geesthacht, Germany ([email protected])7 University of Bremen, Germany ([email protected], [email protected])8 University of Würzburg, Germany ([email protected])9 ENEA, Italy ([email protected])10University of Virginia, Charlottesville, USA ([email protected])11 The University of Arizona, Tucson, USA ([email protected])

Detailed insight into temporal and spatial patterns of climate change over the last centuries is

important for assessing the degree to which late 20th century changes may be unusual in the light

of pre-industrial natural climate variability. The Mediterranean area offers a broad spectrum and

dense network of long instrumental series and natural proxies and documentary information,

making this region ideal for climate reconstructions, as well as the analysis of changes in climate

extremes and socio-economic impacts prior to the instrumental period. Scientific challenges for

future research should include the collection of more, natural proxies especially from the eastern

Mediterranean and the exploration of new climate information from Islamic archives, ship logbooks,

and other documentary sources. Using multivariate calibration of all the proxy data against

instrumental records will provide long reconstructions and uncertainties estimations for the larger

Mediterranean area. The reconstructions will allow studying high and low frequency climate

variability, trends, anomalies, abrupt changes, the relation to important atmospheric circulation

patterns (e.g. NAO, AO) and how Mediterranean climate covaried with central and northern

Europe. It will be of much interest to address the question on the causes and nature of past

Mediterranean climate variability; are they random internal variations or related to any forcing such

as solar or volcanic? Another focus will be the investigation of extreme events such as floodings,

droughts, windstorms, frosts, etc. with socio-economic impact in different Mediterranean areas.

19

Another approach complementary to the reconstructions is the use of Global Climate Model

simulations. GCM paleo simulations can be compared with the results inferred from proxy data.

1.1.1 Introduction

A necessary task for assessing the degree to which the instrumental period is unusual against the

background of pre-industrial climate variability, is the reconstruction and interpretation of temporal

and spatial patterns of climate change in prior centuries. Reconstructions of past climate can also

help inform our dynamical and physical understanding of the relevant processes. To deduce a

year-by-year chronology of pre-instrumental climate changes, we must rely upon high-resolution

‘proxy’ climate indicators from natural archives (corals, ice cores, tree rings, lake sediments,

boreholes and speleothems, e.g. Mann et al. 1998, 2003; Jones et al. 2001; Briffa et al. 2001;

Esper et al. 2002) and documentary evidence (chronicles and historiographies, narratives, annals,

records of public administration and government, scientific writings, monastery records, information

on religious processions and others--e.g. Martin-Vide and Barriendos 1995, Barriendos 1997,

Rodrigo et al. 1999, 2000; Pfister 1999, Glaser 2001, Pfister et al. 2002; Garcia et al., 2003; Brázdil

et al. 2002, 2004).

Figure 2. (left) Weather diary of Wolfgang Haller, January 1573, Zurich (from Pfister 1999) (right)

Flood of September 1862 in Barcelona after removing the walled perimeter (from Amades 1984).

Documentary proxy information provide the only evidence that directly relates to socio-economic

climate impacts, in particular with regard to rare but socio-economically significant natural disasters

(e.g. severe floods, droughts, windstorms, frosts, hailstorms, etc.) prior to the period of

instrumental measurement (e.g. Pfister et al. 2002, Brázdil et al. 2002, 2004). Figure 2 presents an

20

example of a weather diary from Switzerland (Pfister 1999) and a past flood event in Barcelona

1862 (Amades 1984). The Mediterranean area offers a broad spectrum of both natural and

documentary information, both in time and space making this area ideal for climate reconstructions

of past centuries, as well as the analysis of changes in climate extremes and socio-economic

impacts prior to the instrumental period. We first report on reconstructions of climate variables from

documentary and natural data at local or regional scale including climate extremes and the

incidence of natural disasters. We then turn to recent attempts of large-scale multi-proxy field

reconstructions for the Mediterranean. Finally, we comment on the scientific challenges for future

research on past Mediterranean climate variability.

1.1.2 Regional Mediterranean Climate Variability and Extremes Based on

Documentary Evidence

SPAIN. The Spanish historical archives exhibit great potential for inferences into climate variability

at different time-scales and for different territories. García et al. (2003) report on the main archives

and discuss the techniques, strategies and potential to obtain climate-relevant information from

documentary records. Martin-Vide and Barriendos (1995), Barriendos (1997) and Barriendos and

Llasat (2003) used rogation ceremony records for climatic reconstructions for Catalonia. Rodrigo et

al. (1999, 2000, 2001) reconstruct a 500-year seasonal precipitation record for Andalusia, derive a

winter NAO index, and interpret its past variability. Precipitation in the Canary Islands has been

reconstructed through the use of agricultural records for the period 1595-1836 (García Herrera et

al. 2003). It is shown that the NAO correlates negatively with rainfall, even in this southern limit of

the greater Mediterranean area. Barriendos and Martin-Vide (1998), Benito et al. (2003) and Llasat

et al. (2003) investigated flood magnitude and frequency within the context of climatic variability for

the last centuries for central Spain and Catalonia. The authors found evidence for high flood

frequencies in the past similar to present conditions.

FRANCE. The extensive study of Pichard (1999) in Southern France (Provence) has shown that

the 1550-1920 period was characterized by much more frequent floods of the Rhone and Durance

rivers compared to the 20th century. Presence of floating ice on these rivers was not at all seldom

while they are almost absent since one century. The wettest climate of the ‘Little Ice Age’ was, as

for other parts of the Mediterranean (see Xoplaki et al. 2001 for Greece), during the periods 1650-

1710 and 1750-1820.

21

ITALY. Also Italian archives, libraries and museums provide a great number of written historical

sources on different aspects of past climate reaching back more than 1000 years. Camuffo and

Enzi (1992, 1994, 1995) report on extreme events in Italy with specific emphasise on the Late

Maunder Minimum period (1675-1715). A large number of citations, pictorial and literary

representations have been used to study the freezing of the Venetian lagoon since the 9th century

(Camuffo 1987) and its surges (Camuffo 1993, Enzi and Camuffo 1995, Camuffo and Sturaro

2003). Further, Camuffo et al. (2000abc) investigated fire risk, the storm frequency in the Adriatic

Sea, as well as past hailstorms and thunderstorms over the last centuries. Finally, Piervitali and

Colacino (2001) analysed drought events that occurred in western Sicily during the period 1565-

1915 using historical information from the church.

SOUTHERN BALKANS and GREECE. The Balkan Peninsula (Greece, former Yugoslavian

countries, Albania, Bulgaria and Romania) provides rich archives of documentary data. Repapis et

al. (1989) investigated the frequency of occurrence of severe Greek winters based on evidence

from monastery and historical records during the period 1200-1900. They found evidence, that the

coldest periods occurred in the first half of the 15th century, in the second half of the 17th century

and in the 19th century. Grove and Conterio (1994, 1995), Grove (2001) and Xoplaki et al. (2001)

reported on the variability of climate and extremes during parts of the ‘Little Ice Age’.

1.1.3 Natural Proxy-Based Reconstructions of Past Climate over Specific Regions of

the Mediterranean

Many past studies have described the use of tree-ring or 'dendroclimatic' data to reconstruct past

variations in precipitation, temperature, the frequency of extreme droughts, and atmospheric

circulation indices.

Novau et al. (1995) used tree-ring information to reconstruct the climatic conditions in Galicia

(north-western Spain) for the last centuries. Recently, Touchan et al. (2003) used tree-ring data

from southwestern Turkey to reconstruct spring (May-June) precipitation several centuries back in

time. They found, that spring drought (wetness) is connected with warm (cool) conditions and

southwesterly (continental) circulation over the eastern Mediterranean. D’Arrigo and Cullen (2001)

presented a 350-year dendroclimatic reconstruction of February-August precipitation for central

Turkey (Sivas). Touchan et al. (1999) developed a reconstruction of October-May precipitation for

southern Jordan back to 1600. These reconstructions show evidence of multi-year to decadal

rainfall variations over the eastern Mediterranean.

22

Meko (1985) and Chbouki et al. (1995) discussed temporal and spatial variation of Moroccan

drought. Till and Guiot (1990) published a 900-year reconstruction of October-September

precipitation for three different areas in Morocco, indicating a continuous tendency towards a

wetter climate during the 20th century, and drier conditions than present during the 16th, 17th and

18th century. Glueck and Stockton (2001) used climate-sensitive Moroccan tree-ring data to

reconstruct the winter NAO index back to 1429. Tree-ring data have also been used to estimate

seasonal and annual mean temperature at specific Mediterranean areas for the last centuries

(Serre-Bachet and Guiot 1987; Serre-Bachet et al. 1992; Guiot et al. 1988). Briffa et al. (2001)

presented an averaged southern European April-September temperature series back to the 17th

century.

Felis et al. (2000) showed that coral records from the Northern Red Sea provide evidence that

interactions between tropical/extra tropical modes of the global climate system had an important

control on Middle East climate variability. Colder (warmer) conditions go along with more arid (less

arid) conditions in the northern Red Sea and wetter (drier) conditions in the southeastern

Mediterranean. It was shown that during winter the coral record is linked to the Arctic Oscillation

(AO), which controls the advection of colder air from southeastern Europe towards the northern

Red Sea (Rimbu et al. 2001). This finding is consistent with other evidence of a connection

between the Northern Hemisphere (NH) annual modes (‘AO’ or ‘NAO’) and Middle East climate

variability in past centuries (Cullen et al. 2002; Mann 2002).

1.1.4 Large Scale Climate Reconstructions for the Mediterranean

Complimentary in its focus to the local or regional reconstructions of changes in climate and

climate extremes discussed above, large-scale reconstructions of climate fields, employing the

multivariate calibration of the proxy data against instrumental records, allow insight into both the

spatial and temporal details about past climate variations over the Mediterranean region. This

approach of climate field reconstructions provides a distinct advantage over averaged climate

reconstructions for instance, when information on the spatial response to external forcing (e.g.

volcanic, solar) is sought (e.g. Fischer et al. 2004).

Briffa et al. (2002) have used tree-ring maximum latewood density data to reconstruct large-scale

patterns of warm-season (April-September) mean temperature for the period 1600-1887 for the

NH, including the Mediterranean. Mann (2002) used proxy data and long historical and

instrumental records to reconstruct and interpret large-scale surface temperature patterns back to

the mid-18th century for the Middle and Near East. This study suggested that interannual

temperature variability in these regions in past centuries appears to been closely tied to changes in

23

the NAO. Pauling et al. (2003) demonstrated that speleothems from Scotland and tree-ring data

provided the greatest levels of skill in reconstructions of boreal cold-season (October-March)

temperatures in the western Mediterranean, while historical documentary evidence provided the

greatest skill for reconstructions of the northern basin and parts of northern Africa, and Red Sea

coral data provided the greatest skill in reconstructions of the eastern basin. For boreal warm-

season (April-September) temperatures, tree-rings, documentary data and the speleothem proved

to be most important. Thus, large-scale climate reconstructions based on a careful selection of a

combination of temperature/precipitation sensitive proxies from the whole of Europe, including the

Mediterranean provide the most reliable means for reconstructing past regional and seasonal

climate variability. Luterbacher and Xoplaki (2003) have produced multiproxy (instrumental station

series, documentary evidence and single tree-ring data) winter mean spatially-averaged

Mediterranean temperature (Figure 3) and precipitation (not shown) time series back to 1500. They

found several cold relapses and warm intervals as well as dry and wet periods on the decadal

timescale, on which shorter-period quasi-oscillatory behaviour was superimposed. They report also

on the spatial temperature anomaly distribution for cold and warm winter extremes.

Figure 3. Winter (DJF) mean Mediterranean temperature time series from 1500-1995 defined as

the average over 10°W-40°E; 30°N-47°N (thin green line). The winter values for the period 1500-

1900 are the reconstructions, the values from the 20th century are taken from the New et al. (2000)

data set. The thick dark green line is the 9-point low pass filtered time series. The long-term (1500-

1995) winter temperature mean is given in yellow. The warmest and the coldest winters of the

reconstruction period are denoted by black letters (Luterbacher and Xoplaki 2003).

Guiot (1991) used a combination of documentary proxy evidence and natural proxies to provide

annual temperature estimates from 1068-1979 for Europe, including a large part of the

Mediterranean area. He found a significant connection between Northwest Europe and the Central

Mediterranean region during the ‘Little Ice Age’, while the Western Mediterranean region had not

experienced any significant cooling.

24

1.1.5 Scientific Challenges for Future Research on Past Climate

• Collect further high resolution, accurately dated, natural and documentary proxy evidence,

especially in areas with scarce information (North African coastal regions). Archives of the

Islamic world are believed to provide evidence on past weather and climate. Attention

should be paid to logbooks, since they provide relevant climate data over the sea areas.

The abstraction of logbooks for other sea areas is currently under development

(www.ucm.es/info/cliwoc). Logbook information should also be abstracted for the

Mediterranean, since it has historically been a well-travelled sea. In Cyprus, Turkey and the

near East there is potential for more tree-ring series.

• Combine all climate information in a ‘multiproxy’ approach to provide highly spatially and

temporally resolved climate reconstructions for the Mediterranean and sub-regions,

focusing on temperature, precipitation, and other parameters such as potential

evapotranspiration, which is a key variable for agriculture and water-resource

management.

• Estimation of uncertainties of the reconstructions, both in time and space.

• Study of high and low frequency climate variability over the Mediterranean area. Study of

trends, anomalies and abrupt changes for different seasons.

• Relationship between independently reconstructed atmospheric circulation patterns (e.g.

NAO, AO, EU, etc.) and Mediterranean climate. Are the relationships stable through time, if

not, what might be the reasons?

• Can we obtain past information on the state of the Mediterranean Sea and its connection to

the climate in the past?

• Analysis of single (extreme) events and investigations of trends in extremes.

• Relate natural disasters (extremes) from Mediterranean areas to the atmospheric

circulation. Which flow patterns are relevant for climate extremes, do the patterns change

through time both in frequency and in within-type characteristics? (e.g. Jacobeit et al.

2003).

• How does Mediterranean climate co-vary with central/northern Europe?

• Examination of causes and nature of past Mediterranean climate variability, extremes and

rapid climate change. What are the roles of large volcanic eruptions (e.g. Fischer et al.

25

2004), solar variability and contributions from anthropogenic forcing factors in space and

time?

• Examination of how climatic change over the Mediterranean has influenced society and

human activities.

• Comparison of reconstructions of past Mediterranean variability and transient global climate

simulations and regional climate models of the last millennium can bring a twofold benefit:

Firstly, they can be used to validate climate reconstruction methods, which can be tested in

the simulated climate states. Secondly, they can be used to identify the limitations of the

model in reproducing climate variability and in simulating past climate changes (e.g. Zorita

and González-Rouco 2002; González-Rouco et al. 2003; Zorita et al. 2004). They can, in

turn, then be used to establish uncertainties in scenarios of future climate change.

1.2 Present Trends of Mediterranean Climate

Jürg Luterbacher1, Elena Xoplaki1, J Fidel Gonzalez-Rouco2, Ricardo Garcia Herrera2, Eduardo

Zorita3, Jucundus Jacobeit4, Jose Carlos Gonzalez-Hidalgo5, Murat Türkes6, Luis Gimeno7, Pedro

Ribera8, Manola Brunet-India9, Michel Crepon10, Annarita Mariotti11, Pinhas Alpert12

1 University of Bern, Bern, Switzerland ([email protected], [email protected])2 Universidad Complutense, Madrid, Spain ([email protected], [email protected])3 Institute for Coastal Research, Geesthacht, Germany ([email protected])4 University of Würzburg, Germany ([email protected])5 University of Zaragoza, Zaragoza, Spain ([email protected])6 Turkish State Meteorological Service, Ankara, Turkey ([email protected])7 Universidad de Vigo, Ourense, Spain ([email protected])8 Universidad Pablo de Olavide de Sevilla, Spain ([email protected]),9 University Rovira i Virgili, Tarragona, Spain ([email protected])10 LODYC, Universite P.-et-M.-Curie, Paris, France ([email protected])11 ENEA, Italy ([email protected])12 Tel Aviv University, Israel ([email protected])

Instrumental data reveal significant trends of Mediterranean temperature and precipitation at

different time and space scales. For instance, during the last 50 years of the 20th century large

parts of the Mediterranean experienced winter and summer warming. For the same period

precipitation over the Mediterranean decreased, though only partly statistically significant due to

the large variability. These trends, however, differ across regions and periods under consideration

showing variability at a range of scales in response to changes in the direct radiative forcing and

variations in internal modes of the climate system. It is one of the main challenges for future

research to understand the physical processes and causes responsible for these trends. They

seem to be hemispheric to global (such as external forcings and changes in the large-scale

26

atmospheric circulation), anthropogenic as well as local/regional (such as changes in earth surface

and land use, orography). One part for instance will deal with the investigation of the important

large-scale climate modes to explain regional temperature and precipitation variability and trends

for different periods and seasons within the instrumental period using sophisticated statistical

methods. These circulation-climate relationships derived in the instrumental period can then be

used for past climate estimates, seasonal forecasts as well as for assessing future climate

changes in dependence of scenario projections.

Further, analyses of climate model simulations of the 20th century with regional climate models

driven by boundary conditions produced by global general circulation models will be performed

using long integrations (e.g. 50 years) with different forcing factors (e.g. only solar variability and

only changing anthropogenic greenhouse gas concentrations) in order to help to elucidate the

origins of trends.

Apart from interannual to interdecadal considerations, future work will also deal with the analysis of

climate extremes such as heat/cold waves, droughts, floods, etc. for different parts of the

Mediterranean connected with socio-economic impact.

1.2.1 Introduction

Observational studies indicate significant climate trends on different time scales in the Atlantic-

European area, including the larger Mediterranean area. The physical processes responsible for

these trends and changes seem to be hemispheric to global (such as external forcings and

changes in the large-scale atmospheric circulation) as well as local/regional (such as changes in

earth surface and land use). It is one of the main challenges to understand the recent trends and

changes over the Mediterranean region, both in space and time. The following sections summarize

observed Mediterranean climate evolution and gives possible explanations followed by scientific

challenges for future research.

1.2.2 Observed Temperature Trends over the Mediterranean

Giorgi (2002) analyzed the surface air temperature variability and trends over the larger

Mediterranean land-area for the 20th century based on gridded data of New et al. (2000). He found

a significant annual warming trend of 0.75°C, mostly from contributions in the early and late

decades of the century. Slightly higher values were observed for winter and summer. Based on the

same data, Jacobeit (2000) found a distinct summer warming for the 1969-1998 period. The

structure of climate series can differ considerably across regions showing variability at a range of

scales in response to changes in the direct radiative forcing and variations in internal modes of the

27

climate system (New et al. 2001; Hansen et al. 2001; Giorgi 2002). Figure 4 (right) presents the

linear trends of summer surface air temperatures (°C/50yr) for the period 1950-1999. It also shows

the stations, which experienced a significant trend.

Winter (NDJF)

50N

40N

30N

10W 0 40E30E20E10E

-105 -75 -50 -25 0 25

10W 0 40E30E20E10E

Summer (JJAS)

-1.5 -0.5 0.0 0.5 1.5 2.5 3.5

Figure 4. (left) Linear trends of winter (NDJF) station precipitation (mm/50yr) and (right) summer

(JJAS) surface air temperatures (°C/50yr) for the 1950-1999 period. Stations with a significant

trend (90% confidence level, based on the Mann-Kendall test) are encircled (from Xoplaki 2002).

A clear east-west differentiation in Mediterranean summer air temperature trends is visible.

Cooling, though mostly not significant, was experienced over the Balkans, and parts of the eastern

basin. In the other areas, there is a significant warming trend of up to 3°C/50yr. However, the

warming in these regions did not occur in a steady or monotonic fashion. Over most of western

Mediterranean for instance, it has been mainly registered in two phases: from the mid-1920s to

1950 and from the mid-1970s onwards (e.g. Brunet et al. 2001a, 2002, Galan et al. 2001). A

glance at summer air temperature trends for the 1900-1949 period reveals that warming, though

less extreme as in 1950-1999, was experienced in the western basin (not shown). A cooling trend

over 1900-1949 was only prevalent over Libya and Egypt. The trend of winter temperature over

1900-1949 indicate a general cooling in the central basin but a warming in the east and west (not

shown). For the 1950-1999 period, except for the eastern part, there was warming experienced

(not shown). Xoplaki (2002) found a significant cooling trend of Mediterranean winter Sea Surface

Temperatures (SSTs) east of 20°E over the period 1950-1999. The western basin experienced

warming.

1.2.3 Observed Precipitation Trends over the Mediterranean

Recent studies revealed that the 20th century was characterized by significant precipitation trends

at different time and space scales (e.g. New et al. 2001, Folland et al. 2001). Giorgi (2002) found

negative winter precipitation trends over the larger Mediterranean land-area for the 20th century.

Using the same data, Jacobeit (2000) showed for the last three decades some rainfall increases in

autumn (western Iberia and southern Turkey), but dominating decreases in winter and spring.

28

A glance at the Mediterranean regional precipitation trends reveals a more detailed picture of the

general findings. Sub regional variability is high, particularly in areas with contrasted topography

near coastland where also significant trend in variability and monthly totals have been observed

(e.g. Turkes 1996, 1998). The evaluation of regional data series (Figure 4, left) indicate, that trends

in many regions are not statistically significant in view of the large variability (Xoplaki, 2002).

However, significant decreases are prevalent in western, central and the eastern Mediterranean.

For the Mediterranean Sea, precipitation variability has been investigated using gauge-satellite

merged products and atmospheric re-analyses (Mariotti and Struglia, 2002). NCEP re-analyses

show that during the last 50 years of the 20th century Mediterranean averaged winter precipitation

has decreased by about 20%, with the decrease mostly occurring during the period late 1970s to

early 1990s.

1.2.4 Observed Daily Rainfall and Temperature Trends over the Mediterranean

Only few areas have been studied on a daily basis in the Mediterranean because high quality data

are rather scarce (e.g. De Luis et al. 2000). Difficulties exist in determining trends of very rare

events (e.g. Frei and Schär, 2001). One exception is the dense daily rainfall data-base for the east

of the Iberian Peninsula (Romero et al. 1998, 1999) which show successive drying in western

Catalonia and central and western Andalusia for the period 1964-1993. Brunetti et al. (2001ab)

have found a negative trend for the number of wet days and annual rainfall in Italy, while the

heaviest events class interval show a positive trends. Alpert et al. (2002), Brunetti et al. (2001ab),

Goodess and Jones (2002) also report on a tendency to more intense concentration of rainfall to

have occurred along some Mediterranean coastal areas, essentially Italy and Spain. Similar results

were found in two long observatories in north-eastern inland of Spain (Ramos, 2001).

Over the western Mediterranean little change or even an increase of the day/night temperature

differences has been highlighted for the last 130 years (Brunet et al. 2001bc) and for the 20th

century (Brunet et al. 1999, Abaurrea et al. 2001, Horcas et al. 2001). Maximum temperature

increased at larger rates than minimum temperature. This diurnal differential rate of warming,

opposite to the observed on larger spatial scales, has been mainly intensified during the second

half of the 20th century.

1.2.5 Possible Explanations of Trends

Trends in large-scale atmospheric circulation patterns affect trends in Mediterranean temperature

and precipitation. The interaction with topography and land-sea contrasts can produce a variety of

regional responses with different trend signs, although they have the same origin. Xoplaki et al.

(2003) showed that the 300 hPa geopotential height, 700-1000 hPa thickness and Mediterranean

29

SST large-scale fields account for more than 50% of the Mediterranean summer temperature

variability over the period 1950-1999. The most important summer warming pattern is associated

with blocking conditions, subsidence and stability. This mode is responsible for the 0.4°C (0.5 °C)

warming during the period 1950-1999 (1900-1999).

Xoplaki et al (2004) show that around 30% of the October-March precipitation variability can be

accounted for by four large-scale circulation modes. The CCA of Dünkeloh and Jacobeit (2003)

indicates that 76% of the October-March rainfall variability is accounted for by five coupled patterns

with 61% explained variance for the five large-scale circulation modes. The most important mode is

significantly correlated with the NAO and the AO (Arctic Oscillation) pattern. It is connected with

above (below) normal precipitation over most of the Mediterranean with highest (lowest) values at

the western coasts of the peninsulas and lowest (highest) in the southeastern regions. Figure 5

presents standardized values of October-March precipitation anomalies over 110 Mediterranean

stations for the 1900-1999 period and the Gibraltar-Iceland NAO (Jones et al. 1997) with reversed

sign. The correlation between both series is 0.6, suggesting that the North Atlantic climate

variability plays a crucial role in driving long-term trends in the Mediterranean. Further, the NAO

index correlates at 0.72 with a large-scale Mediterranean Oscillation pattern during October-March

(Dünkeloh and Jacobeit 2003).

Figure 5. Gibraltar-Iceland NAO index (Jones et al. 1997) with reversed sign; RR: standardized

average of precipitation anomalies over 110 sites over the Mediterranean region between 1900

and 1999. The time series is shown as a 4-year moving average filter (from Xoplaki et al. 2004).

The recently observed trend towards drier Mediterranean winter conditions is linked to particular

circulation pattern changes including increased pressure south of 45-50°N since the 1970s, a

weakening of the central Mediterranean trough since the late 1980s, and a long-term rising trend in

the Mediterranean oscillation pattern (higher pressure in western and central Mediterranean) being

connected to the AO and NAO (Dünkeloh and Jacobeit, 2003). Though the NAO/AO plays an

important role in driving temperature and precipitation trends in the Mediterranean, its influence

varies through different time periods. Thus, there are other modes which are of relevance for

explaining seasonal sub-Mediterranean climate variability (e.g. Kutiel and Benaroch, 2002;

30

Dünkeloh and Jacobeit, 2003; Xoplaki et al. 2003, 2004) or indirectly through the effect over sea

level pressure patterns (Ribera el al. 2000).

Solar variability and troposphere-stratosphere interaction (Perlwitz and Graf 1995, 2001; Shindell

et al. 1999) as well as ocean dynamics (Sutton and Allen 1997; Rodwell et al. 1999) have also

been suggested as potential sources of variability affecting the North Atlantic climate with impacts

on the Mediterranean regions. Hoerling et al. (2001) found that much of the 500 hPa height

changes in winter are recoverable from tropical SST forcing alone, thereby establishing that

wintertime North Atlantic/European climate changes (dryness over the Mediterranean area) since

1950 are consistent with changes in the tropical SSTs. Recent findings of SST anomaly

experiments by Hoerling et al. (2003) and Hurrell et al. (2003) indicate that SST variations have

significantly controlled the North Atlantic circulation, related to the NAO, with the warming of the

tropical Indian and western Pacific Ocean being of particular importance.

The nature of different rates of rainfall decrease in the east coast of Iberian Peninsula and parts of

Italy might be related to the observed increasing precipitation intensity, owing to a general

enhancement of the hydrological cycle (caused by an increase in surface temperature), and the

reduction of the number of wet days which are consistent with the variation of the atmospheric

circulation (Brunetti et al. 2001 a).

Minimum winter extreme temperatures across Peninsular Spain for the period 1955-1998 indicate

that most of the extremes occurred under six synoptic patterns. A generalised decreasing trend in

the annual frequency of extreme events is detected for most of the studied observatories. Prieto et

al. (2004) showed, that it is due to a non-linear shift in the annual mean minimum temperatures

associated to a generalised warming in the area. An explanatory hypothesis of the differential

diurnal warming observed over the western Mediterranean can be found in Fernandez-Garcia and

Rasilla (2001). They showed an increase of the geopotential height over the region, particularly

intense during the second half of the 20th century. This was associated with increasing solar

radiation, higher maximum temperatures and an intensified radiative lost at night, which have

smoothed rising in daily minimum temperatures.

1.2.6 Scientific Challenges for Future Research

• Generate more reliable and consistent climate datasets (both on daily and monthly basis)

over the entire Mediterranean basin.

• Investigate the change of the annual cycle in Mediterranean temperature and precipitation

through the instrumental period.

31

• Investigate temperature and precipitation trends at different time and spatial scales over the

Mediterranean and sub areas for different periods and seasons.

• What are the main reasons and explanations for the observed trends?

• What is the influence of tropical SSTs on Mediterranean climate?

• Investigation of the important large-scale climate modes to explain regional temperature

and precipitation variability and trends for different periods and seasons within the

instrumental period using sophisticated statistical methods.

• Determine coupled modes of circulation and climate variability at seasonal scale with

respect to both the Mediterranean area and its sub regions.

• Analyze long-term changes in climate extremes (heat/cold waves, droughts, floods, etc.)

over the Mediterranean region.

• Use the circulation-climate relationships from the instrumental period for past climate

estimates, seasonal forecasts as well as for assessing future climate changes in

dependence of scenario projections (e.g. Garcia et al. 2003).

• Analyses of climate model simulations of the 20th century with regional climate models

driven by boundary conditions produced by global GCMs. Long integrations (e.g. 50 years),

with different forcing factors (e.g. only solar variability and only changing anthropogenic

greenhouse gas concentrations) could be of much help to elucidate the origins of trends

(e.g. González-Rouco et al. 2000).

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Zorita E, von Storch H, González-Rouco FJ, Cubasch U, Luterbacher J, Fischer-Bruns I, Legutke

S, Schlese U, 2004: Transient simulation of the climate of the last five centuries with an

atmosphere-ocean coupled model: the Late Maunder Minimum and the Little Ice Age,

Meteorol.

39

2a. Relations Between Climate Variability In The Mediterranean

Region And The Global Tropical Oceans: ENSO, The Indian

And African Monsoons

Pinhas Alpert1, Marina Baldi2, Ronny Ilani1, Shimon Krichak1, Colin Price1, Xavier Rodó3, Hadas

Saaroni1, Baruch Ziv4

1 Tel Aviv University, Israel ([email protected], [email protected], [email protected])2 ISAC – CNR, Italy ([email protected])3 Climate Research Group, University of Barcelona, Spain ([email protected])4 The open university of Israel ([email protected])

2a.1 Priority Issues and Future Challenges

ENSO Impact on Mediterranean (MED) Climate:

• To investigate the role of both transient and stationary waves in the atmospheric response in

the Atlantic, Mediterranean & Europe to ENSO-events, separately for La Niña and El Niño

events.

• To study the role of mid-latitude oceans in responding to tropical forcings and its effect on the

MED climate, observationally and by modelling simulations.

• To apply advanced statistical techniques to address local phenomena, such as the new Scale-

Dependent Correlation (SDC) technique.

Indian Monsoon Impacts on MED Climate:

• To study teleconnections for diurnal, seasonal and decadal time scales and to evaluate the

range of influence of the Asian Monsoon over the entire MED basin.

• To study the detailed structure of the summer circulation over the EM associated with extreme

episodes, such as heat waves or exceptional rain events in the EM.

• To study the long-term trends in a variety of meteorological variables.

• To perform a climatic study of continental polar outbreaks over the MED.

• To assess the statistical relationship between the variations in the EM rainfall distribution and

intensity and the long-range variations in the characteristics of the air mass transport

associated with the Indian Ocean Dipole.

40

African Monsoon Impacts on the Climate of the MED:

• To study the teleconnections between the summer climate in the MED and the African

Monsoon through numerical simulations.

• To perform numerical simulations with the Regional Model on different spatio-temporal scales

on a domain including Europe, the MED and Africa. This includes the effects of the SST in the

Gulf of Guinea on the MED climate variability and the influence of the MED SST on the climate

variability of the North Africa.

• To perform time-slice experiments for future climate evolution, using the regional model,

according to various scenarios, simulating the evolution of the Hadley cell.

• To develop close link between CLIVAR VACS (Variability of the African Climate System) and

AMMA (African Monsoon Multidisciplinary Activities).

Tropical Intrusions into the MED Basin:

• To define the general mechanisms associated with tropical intrusions into the EM, i.e., the Red

Sea trough and Tropical Plumes.

• To find the role of the Red Sea trough and the tropical plume in the general circulation of the

MED, in particular moisture and angular momentum transport.

2a.2 ENSO Impact on Mediterranean (MED) Climate

2a.2.1 ENSO and Eastern Mediterranean (EM) Rainfall

Yakir et al. (1996) and Price et al. (1998) showed significant connections between ENSO events

and winter rainfall in Israel, both indicate increased rainfall occurring in El Nino winters. Price et al.

(1998) also demonstrated that La Nina years were associated with below normal rainfall. The last

rainy winter in Israel, coinciding with Nino event, supports the above. The analysis in Israel was

extended to the Jordan River discharge, used as a proxy for regional rainfall, since the stream flow

entering the Sea of Galilee is dominated by regional rainfall. The seasonal stream flow in the

Jordan River is significantly correlated (r~0.6) with the seasonal NINO3.4 temperatures. This

implies that the tropical Pacific temperature oscillations can explain approximately 36% of the inter-

annual variability in winter rainfall in North Israel. It is hypothesized that the reason for this strong

connection is related to the position of the winter jet in the EM. Israel is located at 30N, exactly the

mean latitude of the winter jet. Small shifts, in the order of ~1 deg, in its mean position can have a

major impact on the storm tracks, hence on the rainfall amounts. During El Nino/La Nina years

41

meridional shifts of the jet in the EM have been observed. However, the intensity of the ENSO

events is not directly related to the intensity of the rainfall anomalies in Israel. This is one of the

reasons the correlation coefficient is only 0.6. However El Nino/La Nino years have been wet/dry

for 75% of the ENSO events in the last 30 years. Stream flow data in the Jordan River are only

available since the end of the 1960s. However, since individual rain gauge measurements

watershed is highly correlated (r~0.9) with the catchment's-integrated stream flow, it is possible to

go back to 1922. The ENSO signal appears in the rainfall data only after the mid 1970s. It is

puzzling as to why these correlations are observed only in the recent record. This may be a result

of the changes in the frequency and intensity of ENSO events since the mid 1970s. Trenberth and

Hoar (1997) have shown that since the mid 1970s there has been a significant increase in the

frequency of El Nino events relative to La Nina events, and the intensity and period of these events

has also increased. It has also been suggested that there may have been a shift in the global

climate system during the 1970s, which may have resulted in a stronger Pacific-mid-latitude link

during the past two decades (Wuethrich 1995).

2a.2.2 El-Nino and Extreme Mediterranean Rainfall

Alpert et al (2002) calculated the relative contributions of 6 daily rainfall intensity categories to the

annual rainfall amounts during 1951-1995, over Spain, Italy, Cyprus and Israel. Linear as well as

monotone non-linear (Spearman’s) time test show significant increases in heavy daily rainfall in

spite of decreases in the annual totals. For instance, in Italy, torrential rainfall (%) above 128 mm/d

increased by a factor of 4! in 1951-1995. Most interesting the torrential rainfall peaks were

observed in the El-Nino years.

2a.2.3 Transient and Stationary Waves Approach

Previous work has shown an ENSO-impact during boreal winter with a trough (ridge) over southern

Europe during El Niño (La Niña) events accompanied by more (less) cyclones reaching the

Mediterranean (MED) region. That is, both the mean flow and sub-seasonal variations of the flow

are affected by ENSO. In particular, the sub-seasonal variations tend to feedback on the

anomalous mean flow. However, the impact in the Atlantic and Europe, and the MED region in

particular, appear to be more robust during La Niña than during El Niño events. Previous work from

the SINTEX-project indicated that the dominating mode of interaction - resembling the NAO - is

only related to La Niña but not to El Niño events. Further, these modes - though defined in the

Atlantic & Europe - appear to be connected to the N Pacific and N America. This suggests that the

transient eddies are also important in “transporting” the ENSO-response from the latter regions into

the Atlantic & Europe. The insight gained may improve the prospects of seasonal prediction in the

42

Atlantic/European region. Modelling experiments can cope with a complex response to ENSO

through the alteration of mid-latitude internal modes of variability (e.g., NAO, EATL-WRU, etc.), in

particular with respect to future scenarios (e.g. Timmermann et al. 1999).

2a.2.4 ENSO and Mid-Latitudes Relationship

Rodó et al. (1997) investigated the signatures of ENSO in Spanish precipitation and stated a

coherent decrease in MAM(+1) following El Niño events, in accordance to that stated in pioneering

studies (Ropelewski & Halpert 1987, Kiladis & Díaz 1989), and later confirmed and extended by

van Oldenborgh et al. (2000) and Mariotti et al. (2002). This coherence appeared to increase in the

second half of the 20th century. Regarding ENSO-signatures in the NAE sector, Rodó (2001)

highlighted the difficulty in isolating ENSO signals using common statistical techniques, mainly due

to their spiky nature with respect to the dominating mid-latitude dynamics. Their importance for

MED climate might be high, though only for selected intervals and vanish elsewhere. Rodó (2001)

showed this occurrence for SST anomalies in the western MED basin. The possibility of an ENSO

influence through perturbations of the Atlantic Walker circulation was also highlighted by Rodó

(2001), who stated the importance of a weak Atlantic Hadley cell as a response to anomalous

warming in the eastern tropical Pacific (in accordance to Sutton et al., 2000; and Saravanan and

Chang, 2000). Their results suggest that a fraction of the inter-hemispheric variability in the tropical

Atlantic is forced by way of a tropical atmospheric bridge (Lau & Nath 1996, Klein et al. 1999).

2a.2.5 Recommended Future Research

• To investigate the roles of both transient and stationary waves in the atmospheric response in

the Atlantic, Mediterranean & Europe to ENSO-events. In particular, their role in initiating the

anomalies in the mean flow and in determining the specific characteristics during individual

events.

• To investigate the difference between the atmospheric responses separately to La Niña

events, which is much less robust with respect to the El Niño, starting from a description of the

interactions between the transient eddies and the mean flow over the affected regions (since

they are essential for determining anomalies in the mean flow).

• Advanced statistical techniques will be applied to address local phenomena, such as the new

Scale-Dependent Correlation (SDC) technique (Rodó 2001, Rodó et al. 2002, Rodríguez-Arias

& Rodó 2003), due to the complex fashion in which ENSO interacts with the mid-latitudes.

• The role of mid-latitude oceans in responding to atmospheric forcing of tropical origin (Lau &

Nath 2001) and its effect on the MED climate will be studied, both observationally and by

modelling simulations. Improvement is expected to be achieved in the resolution and accuracy

of observational studies with the use of a denser, homogeneous set of records. Various

43

scenarios of future ENSO frequency and intensity related to climate change will be explored for

assessing the relation to MED climate variability and extremes.

2a.3 Indian Monsoon Impacts on MED Climate

2a.3.1 Indian Monsoon and the Eastern Mediterranean

The study of the teleconnections between central Asia and the EM deals with the extreme

seasons, the summer and the winter, separately.

Rodwell and Hoskins (1996) showed that the Asian Summer Monsoon dominates not only Mid-

Asia, but also the EM. In numerical simulations they pointed at the linkage between the

appearance of the semi-permanent subsidence region over the EM and the onset of the Monsoon.

The climatic regime and the dynamic factors governing the EM in the summer season and their

relationships with the Asian Monsoon were analyzed by Ziv et al. (2003), who found significant

correlation on the inter-diurnal time-scale. They identified a circulation connecting the upward

motion maximum over the Himalaya with the downward motion over the EM. Raicich et al. (2003)

studied the relationship between the Asian and African Monsoon systems and found a high

correlation between the intensity of each of them and the pressure distribution over the MED on

the inter-annual time-scale.

The Indian summer Monsoon index is recorded for almost 200 years, while records of winter rain in

Israel are relatively ‘younger’; the longest record used is of 118 years in Jerusalem. The overall

correlation between these two indices was found to be only -0.3 (for 118 years). However, in 73

years (62%) the indices sign were opposite. For extreme summer seasons, in which the index

deviates by over 1.3 STDs, the correlation increases to -0.56 (Alpert et al., 2003). Similar results

were found for other relatively long-record of rainfall stations in Israel. This illustrates the potential

of the Indian Monsoon as a predictor for Israeli rainfall in the following winter season.

While the role of the Tropical Atlantic Variability (TAV), ENSO and associated changes in SST over

the tropical Pacific and Atlantic oceans in the tropics, especially in the Indian monsoon, have been

widely investigated, the role of the effect of the Indian Ocean is not well understood. The existence

of the Indian Ocean Dipole (IOD) mode was demonstrated by Saji et al (1999), Webster et al.

(1999) and Andersen (1999). A respective index was determined, though no statistical relationship

between it and the monsoon rains has been established. It is suggested that the variations in the

EM rainfall, distribution and intensity during the last decade are associated with variations in the

characteristics of the air mass over the Indian Ocean via its transport toward the EM.

When the winter regime over the entire MED is considered, the focus is given to the Rossby waves

and other extra-tropical factors, such as the NAO, as the dominating features. However, some

44

attention has been given to continental polar outbreaks associated with the Asian winter Monsoon

(e.g. Saaroni et al. 1996).


• To study teleconnections of the Indian Summer Monsoon with the EM for different time

scales, i.e., diurnal, seasonal decadal, attempt to evaluate the range of influence of the

Asian Monsoon over the entire MED basin.

• To study the long-term trends in various variables, as performed for summer

temperature, Saaroni et al. (2003), in relation with long-term trends in the Asian Monsoon

features along the entire year.

• To study the detailed structure of the summer circulation over the region prior to and

during extreme episodes in which the EM undergoes heat waves or exceptional rain

events.

• To incorporate into the seasonal prediction scheme for the Israel winter rainfall, data

about the summer Asian Monsoon.

• To validate the suggested linkage between the Indian Ocean processes and the EM

weather.

• To develop a climatologic basis for continental polar outbreaks events over the MED.

This includes both synoptic detailed analyses and statistical study.

• To assess the statistical relationship between the variations in the EM rainfall amount,

distribution and intensity and the long-range variations in the characteristics of the air

mass transport associated with the Indian Ocean Dipole.

2a.4 African Monsoon Impacts on the Climate of the MED

2a.4.1 Background

The climatic variables in the various parts of the MED are correlated with each other as well as

with external circulations. For instance, the MED SLP oscillation (MO), i.e., the difference between

its western and eastern parts, is correlated with precipitation. In the winter, a fundamental role is

played by the NAO index, whereas in summer the regional Hadley cell was found to be correlated

45

with the conditions over the MED. There are also evidences for teleconnections with the Asian

Monsoon and with the Sahel precipitation. The SLP is correlated to the precipitation indices of

these 2 systems negatively in the EM and positively in the Western MED. The relevant governing

mechanisms have been studied by several authors (see Baldi et al., 2002 for an extended

bibliography), as well as the influence of the position and the strength of the Hadley cell (Dima and

Wallace 2003).

Focusing on the summer case, Chen et al. (2002), showed evidence for strengthening of the

tropical general circulation in the 1990s, and in particular the West Africa monsoon, reaching its

northernmost extension in August, when the ITCZ, after the abrupt shift at the end of June and

further slow northward migration, reaches its northernmost location (Sultan and Janicot 2000,

2003). Important mechanisms, such as heat and moisture advection in North America and Asia

and anomalously high values of the surface albedo in northern Africa, limit a further extension

towards northern latitudes (Chou & Neelin 2003, Rodwell & Hoskins 1996, 2001). The two regimes,

the dry and hot summers in the MED and the monsoon regime over West Africa, are strictly

correlated; interactions and feedback mechanisms between the two are not only possible, but also

evident (Rowell 2003, Baldi et al. 2002, 2003a, 2003b).

Ziv et al. (2003), in their study of the summer regime, found a signature of the Hadley cell over

eastern North Africa, connecting the EM with the African Monsoon. The relationship between them

is manifested by a significant correlation between the ascendance in 15-20°N latitudes and the

descendance in 30-40°N latitudes. This, together with the correlation between the EM subsidence

and the Asian Monsoon was further validated through correlating the inter-diurnal variations of the

vertical velocities in the two Monsoon systems, yielding r = 0.33, in spite of the ~6000 km distance.


• To study the teleconnections between the summer climate in the MED and the African Monsoon

through numerical simulations. The major tools will be the NCEP-NCAR and ECMWF

reanalyses, historical time series of atmospheric parameters in Southern Europe, Regional

numerical model(s), scenarios for future climate produced by global climate models like from the

Canadian Centre for Climate Modelling and Analysis (CCCma), and gridded precipitation data

provided by the Global Precipitation Climatology Project.

• To perform numerical simulations with the Regional Model on different time-space scales on a

domain including Europe, the MED Basin and the northern part of the African Continent north to

the Gulf of Guinea. The effects of the SST variability in the Gulf of Guinea on the climate

variability in the MED should be assessed using an approach similar to that presented by Vizy

and Cook (2001, 2002). In turn, the influence of the MED SST on the climate variability of the

North African Region should be studied.

46

• To perform time-slice experiments for future climate evolution, using the regional model,

according to different available scenarios. Since the phenomena are embedded in the large

scale circulation and in particular in the Hadley cell circulation, therefore a mathematical model

of the evolution of the Hadley cell should be elaborated.

• To study the linkages between MED climate CLIVAR VACS (Variability of the African Climate

System) and AMMA (African Monsoon Multidisciplinary Activities).

2a.5 Tropical Intrusions into the MED Basin

2a.5.1 Background

Rains in the Mediterranean basin take place mainly during winter, most of which is associated with

Mediterranean baroclinic cyclones. However, processes originating from tropical regime are also

significant in its eastern part (Krichak et al. 1997a,b, Krichak & Alpert 1998, Dayan et al. 2001,

Kahana et al. 2002, Ziv et al., 2004) and along its western part, in north western Africa (Knippertz

et al., 2003). The Red Sea Trough (RST) is one of the impressive manifestations of mid-latitude-

tropical interactions in the EM especially during autumn and spring. The intensity and duration of

rain-spells there highly depend on the interactions between the upper and lower-tropospheric jets

as well as their positioning and orientation. Specific jet characteristics stimulate development of

meso-scale convective complexes and cyclogenesis. Due to turbulence associated with strong

wind shear, tropopause folding may allow intrusions of the stratospheric air into the troposphere. It

was recently shown that frequencies of RST intrusions into the EM, have nearly doubled since

1970 from about 50 d/y to about 100 d/y (Alpert et al, 2004a,b)

Another type of rainstorms originating from the tropics is associated with "tropical plumes". This is

a long cloud band that extends from the ITCZ down to 300-400N latitude, accompanied by a

pronounced trough in the Subtropical Jet to its west combined with a ridge to the east, while no

common distinct system at the surface or at the 500-hPa level, was found. Ziv (2001) found that

prior to such type of rainstorm the tropical plume, extending toward the subtropics, injects moisture

of tropical origin that is captured by the STJ, and if a pronounced trough develops there, extensive

stratified cloudiness and widespread rains result. Zangvil and Isakson (1995) found in a rainstorm

of the same type that the vertically integrated moisture convergence reached 1.8 mmh-1 over

Israel, mostly above the 750 hPa level. Dayan and Abramski (1983) found an abnormal feature in

the STJ’s structure, i.e., a reversed position of its axis that leads to the formation of a large and

humid warm air mass up to very high levels in the atmosphere above the Middle-East.

47


• To define general mechanisms of tropical intrusions into the EM of the Red Sea trough and

the tropical plume.

• To find the role of the Red Sea trough and the tropical plume in the general circulation of the

MED. In particular in transport of moisture and angular momentum.

• To study the physical mechanisms for the recent increase in tropical intrusions into the MED.

2a.6 References

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systems: Application to the EM climate change. International Journal of Climatology (in

revision).

Alpert, P., I. Osetinsky, B. Ziv and H. Shafir.. 2004. A new seasons definition based on the

classified daily synoptic systems: An example for the EM. International Journal of

Climatology (in revision).

Alpert, P., T. Ben-Gai, A. Baharad, Y. Benjamini, D. Yekutieli, M. Colacino, L. Diodato, C. Ramis,

V. Homar, R. Romero, S. Michaelides and A. Manes, 2002. The paradoxical increase of

Mediterranean extreme daily rainfall in spite of decrease in total values. Geophys. Res. Lett.,

29, 11, 31-1 – 31-4, (June issue).

Alpert, P., R. Ilani, A. da-Silva, A. Rudack and M. Mandel, 2003. Seasonal Prediction for Israel

Winter Precipitation based on northern Hemispheric EOF. MERCHAVIM special issue for

Prof. A. Bitan (accepted).

Andersen D., 1999. Extremes in the Indian Ocean. Nature, 401, 337-339.

Baldi, M., A., Crisci, G.A., Dalu, G., Maracchi, F., Meneguzzo and M. Pasqui, 2002. Mediterranean

climate and its connections to regional and global processes. Proceedings of the First Italian

IGBP Conference Mediterraneo e Italia nel Cambiamento Globale: un ponte fra scienza e

societa’ Paestum (Salerno) 14-16 November 2002

Baldi, M., V. Capecchi, A., Crisci, G.A., Dalu, G., Maracchi, F., Meneguzzo and M. Pasqui, 2003a.

Mediterranean summer climate and its relationship to regional and global processes.

Proceedings of the Sixth European Conference on Applications of Meteorology, Rome, 15-19

September 2003.

Baldi, M., V. Capecchi, A., Crisci, G.A., Dalu, G., Maracchi, F., Meneguzzo and M. Pasqui, 2003b.

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Chen, J., Carlson, B.E. and A.D. Del Genio, 2002. Evidence for strengthening of the tropical

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Chou C. and J.D. Neelin, 2003. Mechanisms limiting the northward extent of the northern summer

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middle-east: synoptic and mesoscale convection analysis. Theor. Appl. Clim., 69, 103-122.

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Southern Oscillation. J. Climate, 2, 1069-1090.

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Knippertz, P., A. H. Fink, A. Reiner and P. Speth, 2003. Three late summer/early autumn cases of

tropical-extratropical interactions causing precipitation in northwest Africa. Mon. Wea. Rev.,

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rainfall and the Mediterranean. Int. J. Climatol., 23, 173-186.

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Rodó, X., 2001. Reversal of three global atmospheric fields linking changes in SST anomalies in

the Pacific, Atlantic and Indian oceans at tropical latitudes and midlatitudes. Clim. Dyn., 18,

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the present century: relationships with the NAO and the ENSO. Clim. Dyn., 13, 275-284.

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with El Niño-Southern Oscillation. Mon. Wea. Rev., 115, 1606-1626.

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tropical Indian Ocean. Nature, 401, 360-363.

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onset of the summer monsoon. Journal of Climate (in press)

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Atlantic region. J. Climate, 13, 3261-3284.

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Vizy, E. K., and K. H. Cook, 2001. Mechanisms by which Gulf of Guinea and Eastern North Atlantic

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Dynamics in the Indian Ocean during 1997-98. Nature, 401, 356-360.

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Yakir, D., S. Lev-Yadun and A. Zangvil, 1996. El Nino and tree growth near Jerusalem over the last

20 years. Global Change Biology, 2, 101-105.

Zangvil, A. and A. Isakson, 1995. Structure of the water vapor field associated with an early spring

rainstorm over the eastern Mediterranean. Isr. J. Earth Sci., 44, 159-168.

Ziv, B. 2001. A subtropical rainstorm associated with a tropical plume over Africa and the Middle-

East. Theor. Appl. Clim., 69, 1/2, 91-102

Ziv B., H. Saaroni and P. Alpert, 2003. The factors governing the summer regime of the Eastern

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southern Israel: Synoptic scale analysis. Meteorology and Atmospheric Physics (in press).

51

2b. Relationships Between Climate Variability in the

Mediterranean Region and Mid-Latitude Variability

Eduardo Zorita1, Ricardo Trigo2, Simon O. Krichak3, Jucundus Jacobeit4, Jon Saenz5, Fidel

Gonzalez-Rouco6, Xavier Rodo7

1 GKSS Research Centre, Germany ([email protected])2 University of Lisbon, Portugal ([email protected])3 Tel Aviv University, Israel ([email protected])4 Institute of Geography, University of Würzburg, Germany ([email protected])5 University of the Basque Countries, Spain ([email protected])6 Universidad Complutense, Madrid, Spain ([email protected])7 Climate Research Group, University of Barcelona, Spain ([email protected])

2b.1 State-of-the-Art.

Since the pioneering work by Lamb and Peppler (1988) on the influence of the North Atlantic

Oscillation on Moroccan precipitation a great deal of effort has been made to characterize the

influence of mid-latitude variability on the climate of the Mediterranean region. This region is

located at the southern limit of the North Atlantic storm tracks, so that interannual shifts in storm

tracks can lead to remarkable anomalies of precipitation and, to a lesser extent, of temperature.

This is specially observed in the cold season in the Western Mediterranean, but it has also a

signature clearly felt in the Eastern Mediterranean as well. One factor that complicates this simple

picture is the complicated orography of virtually all regions surrounding the Mediterranean basin,

that can modulate and even distort climate anomaly patterns that otherwise would be

geographically much more homogenous. The interplay between orography and thermal contrast

between advected Atlantic air masses and Mediterranean temperatures can produce at the end of

the summer season violent precipitation extremes. The main lines of research so far have been

directed at winter half-year precipitation and summer temperature.

Most of the analyses to date have been focused on one important extra tropical factor modulating

the Mediterranean climate in wintertime- the North Atlantic Oscillation, the meridional pressure

gradient along the North Atlantic. It is strongly related to the paths followed by extra tropical

cyclones and to the intensity of the North Atlantic westerlies. Its influence on Mediterranean

precipitation has been widely documented (Hurrell, 1996, Dai et al, 1997). Figure 6 shows the

interannual correlation maps between the North Atlantic Oscillation index and winter precipitation

and temperature in the Mediterranean basin, which clearly shows that a negative NAO state is

responsible for positive precipitation anomalies over most land areas in this region. This

relationships also holds to a large extent for longer time scales (Rimbu et al, 2001), so that the

multiyear drought periods periodically suffered in the Western Mediterranean basin can be traced

52

back to anomalous and persistent excursions of the NAO index towards positive values. Also the

long-term trend towards drier conditions can be explained by trends in large-scale atmospheric

circulation patterns ( Dünkeloh and Jacobeit, 2003).

Figure 6. Correlation between the NAO index and Mediterranean precipitation (left) and air-

temperature (right) in the cold season (December-February), derived from the CRU NAO index and

the NCEP reanalysis.

The high correlation between NAO index and precipitation stresses the importance that a possible

seasonal prediction of the state of the NAO could have on seasonal climate prediction in the

regions near the Mediterranean (Eshel et al., 2000; Rodriguez-Fonseca and Castro, 2002). At still

longer time scales, trends in winter precipitation have been linked to trends in the track of most

intense cyclones originating in the North Atlantic, that have become less frequent in the last

decades (Trigo et al, 2000; Hurrel and van Loon, 1997, Hurrel, 1995). The mechanisms that

explain the connection between NAO and Mediterranean rainfall is also well known. In the Western

Mediterranean the cyclones entering the basin from the North Atlantic are, together with regional

cyclogenesis, one of the main sources of winter rainfall (Ulbrich et al. , 1999; Rogers, 1997) ; in

the Eastern Mediterranean, the situation seems to be somewhat more complicated. In this region

the North Atlantic atmospheric circulation influences the vertical stability of the atmosphere by

temperature advection (Eshel and Farrel, 2000; Xoplaki et al, 2000), but internal eastward moisture

advection within the Mediterranean basin also contributes to modulate Eastern Mediterranean

rainfall (Fernández et al, 2003). More work is needed to achieve a complete understanding and

prioritization of the relevant processes.

Nevertheless, the NAO is not the only player ( e.g. the NAO has been found to play a secondary

role in winter precipitation along the North Iberian coast, Sáenz et al., 2001) and the importance of

other extra tropical modes, e.g. the Eastern Atlantic pattern should not be ignored. The EA pattern

exerts a strong influence on the advection of more humid or warmer air masses to certain regions

(Fernandez et al, 2003). This pattern is also responsible for the modulation of precipitation in the

53

Eastern Mediterranean (Krichak et al., 2002) and to describe much of precipitation anomalies in

the whole basin that cannot be ascribed to the NAO (Quadrelli et al, 2001). .

Although the tropical connections are discussed in more detailed section 2a, we could mention

here that a healthy and unclosed debate on the influence of the indirect effect, via extratropical

modes, of El Nino-Southern Oscillation (ENSO) on precipitation in the Mediterranean region is

going on :some authors report an influence on winter rainfall of ENSO through the NAO (Ribera et

al, 2000), but perhaps only in areas where the NAO influence is weaker (Rodó et al., 1997) or in

the spring or autumn season (Mariotti et al, 2002). Others find no influence of ENSO on winter

rainfall (Quadrelli et al, 2001).

An outstanding problem in Western Mediterranean rainfall is also the intraseasonal distribution of

daily precipitation and the occurrence of catastrophic torrential rains. These tend to occur in the

autumn season along coastlines with heavy orography. Very little work has been done to link the

probability of these events to large-scale extra tropical circulation (Valero et al, 1997), although

some analysis of frequency and intensity of Mediterranean cyclones (Trigo et al, 2002) and

modelling studies for individual events have been performed (Homar et al, 1999; Pastor et al,

2001; Romero et al, 1999). In the last decades a tendency to more intense concentration of rainfall

seems to have occurred along some Mediterranean coastal areas , essentially Italy and Spain,

(Alpert et al, 2002;Brunetti et al, 2001; Goodess and Jones, 2002), but this does not seem to be

the case for inland areas. Certainly more work is needed to link event modelling, long-term

modelling and observations analysis with the goal of a coherent long-term climatological

perspective.

Analysis of Mediterranean cold season temperature has not been so intense as precipitation,

probably reflecting the relevant role of water availability in this region. Cold season temperature is

also affected by extra tropical atmospheric circulation patterns, although the effect seems to be

weaker than for precipitation, and also weaker in the Western Mediterranean. Figure 7a shows for

the cold season a high correlation between NAO and temperatures variations over Europe and

North Africa, whereas the Mediterranean basin is only a transition region with poor correlation. In

the Eastern Mediterranean a positive NAO is related to colder than normal temperatures (Ben-Gai

et al, 2001; Hasanean, 2001). Stronger temperature signals seem to emerge at somewhat longer

timescales than interannual (Ben-Gai, 2001). In the Western Mediterranean the link between the

NAO and temperature has been found to be non-linear and non-stationary in time (Pozo-Vazquez

et al., 2001). Spectral characteristics of the NAO time series are not replicated in the temperature

evolution. In the case of Western Mediterranean temperature, other modes seem to be more

important, e.g. the Eastern Atlantic pattern, and contrary to precipitation, transport by baroclinic

eddies is not a controlling mechanism (Sáenz et al., 2001).

54

Figure 7. Leading Empirical Orthogonal Functions of the Mediterranean air-temperature in summer

(June-August), derived from the NCEP reanalyses

In the summer season, on the other hand, temperature variability has received more attention,

reflecting the widespread perception of heat-stress as a crucial climate factor on the human

environment. Half of the variance of summer temperature variability can be explained by two

variability modes (Xoplaki et al., 2003; Figure 7 shows the two leading Empirical Orthogonal

Functions of the summer Mediterranean air temperature) the first one reflects uniform temperature

anomalies in the whole basin whereas the second displays an east-west dipole pattern. The

existence of both modes can be explained as a response of air temperatures to the location of

highs and lows associated to planetary waves, connected to blocking situations, atmospheric

stability and presumably stronger radiative heating. Trends in the large-scale driving patterns are

especially relevant since they may enhance or damp the warming caused by increasing

greenhouse gas concentrations. However, to our knowledge, work has just beginning in this

direction.

Relevant questions in this context are the possible changes of these extra tropical modes under

global climate change. The results to date seem to indicate that the so called Annular Modes-the

Arctic Oscillation and the Antarctic Oscillation- to which the NAO is linked will tend to become more

intense in the future (Guillet, 2002), although the ratio signal/variability could not very large (Zorita

and Gonzalez-Rouco, 2002). In a more regional basis some work has been reported for the

Western Mediterranean (Gonzalez-Rouco et al, 2000) and the Balcans (Busiuoc et al, 1999).

55

2b.2 Some Ideas for Future Research

Many studies have underlined the role of several teleconnection patterns (NAO, EA, EA/WR) over

different parts of the Mediterranean basin. However, in general it is not clear whether there are

significant differences in the physical mechanisms for the generation and dissipation of these

planetary regimes, or whether the only difference lies simply in the geographical location of their

centres of action. Furthermore, the underlying physical mechanisms (moisture and enthalpy

advection, cyclogenesis and storm tracks, vertical stability, radiation and cloud cover, oceanic

process etc..) that give rise to these statistical connections are not always completely understood.

This question is threefold relevant:

• in the context of changes in the intensity of these circulation patterns under global climate

change. Only with a sufficient understanding of the physical mechanisms will it be possible to

estimate the effect of changes of extra tropical modes on the Mediterranean climate;

• For understanding the variability at decadal time scales (where persistent deviations of

atmospheric indices have been observed), and estimate their influence on low-frequency

changes in temperature and precipitation, in the frame of other possibly competing effects,

such as land use changes.

• seasonal climate prediction would greatly benefit from research in this direction. A substantial

fraction of the climate variability of the Mediterranean basin may be explained in terms of a few

planetary flow regimes. Their predictability, though, is not well quantified, and their origin and

possible relationship with oceanic process is very uncertain. Possibly, in order to make

accurate predictions of variables like precipitation, some kind of downscaling method would be

necessary to fill the gap between the large and the regional scales.

Concerning daily precipitation, most analysis has focused on limited areas in the Western

Mediterranean. Furthermore, due to the large land-sea contrasts and the steep orography

surrounding the Sea (Alps, Pyrenees, Carpatians...), the use of high-resolution climate data would

be very interesting in the Mediterranean area. A very useful tool to advance the state of

knowledge would be the production of high quality, high coverage, data sets. This precondition

could be fulfilled by:

• Generating observational data products with smaller spatial resolution than currently available

(30" of arc) over most part of the basin for at least the past 50 years, due to the large spatial

density of observing sites in some areas and .

• The generation of pseudo-observational data with the help of high-resolution regional climate

simulations driven by available coarse-resolution global meteorological reanalysis (.e.g.,

NCEP, ERA). To date, the number of re-analysis with high resolution and available for periods

56

of climatic importance is very limited (the HYPOCAS project seems to be the only one currently

available). Such "regional reanalysis" could play a similar pivotal role than the global reanalysis

are now playing in climate research.

The knowledge obtained from the analysis of observational or pseudo observational data set could

be extended in two directions:

• the analysis of climate model simulations regarding the reproducibility of the main

characteristics of the extra tropical modes, including its impact on the precipitation field and

cyclonic activity (Osborn et al., 1999; Trigo and Palutikof, 2001, (Ulbrich and Cristoph, 1999).

This work should be continued and intensified with the aim of determining to what extent

climate models yield a realistic picture of the variability in the present climate and quantifying, if

possible, the amount of expected regional climate change that can be ascribed to future rends

in extra tropical modes, since this modes will probably be responsible for regional differences in

the future climate..

• the analysis of empirically reconstructed fields of the last centuries, thus establishing a level of

centennial natural variability, that can be serve as a perspective for changes simulated with

climate models.

2b.3 References

Alpert P; Ben-Gai T; Baharad A; Benjamini Y; Yekutieli D; Colacino M, Diodato L; Ramis C; Homar

V; Romero R; Michaelides S; Manes A. , 2002. The paradoxical increase of Mediterranean

extreme daily rainfall in spite of decrease in total values. GEOPHYSICAL RESEARCH

LETTERS, Vol 29, Iss 11, art. no.1536

Ben-Gai T; Bitan A; Manes A; Alpert P; Kushnir Y, 2001. Temperature and surface pressure

anomalies in Israel and the North Atlantic Oscillation. THEORETICAL AND APPLIED

CLIMATOLOGY, Vol 69, Iss 3-4, pp 171-177

Brunetti M; Colacino M; Maugeri M; Nanni T. Trends in the daily intensity of precipitation in Italy

from 1951 to 1996, 2001. INTERNATIONAL JOURNAL OF CLIMATOLOGY, Vol 21, Iss 3,

pp 299-316.

Busuioc, A., H. von Storch and R. Schnur, 1999: Verification of GCM generated regional

precipitation and of statistical downscaling estimates. - J. Climate, 12: 258-272

Dai A; Fung IY; DelGenio AD., 1997. Surface observed global land precipitation variations during

1900-88. JOURNAL OF CLIMATE, Vol 10, Iss 11, pp 2943-2962

57

Dünkeloh A AND Jacobeit J.. Circulation Dynamics of Mediterranean Precipitation Variability 1948-

1998. Int. J. of Climatology, in press.

Eshel G; Cane MA; Farrell BF. , 2000. Forecasting Eastern Mediterranean dorught. MONTHLY

WEATHER REVIEW, Vol 128, Iss 10, pp 3618-3630

Eshel G; Farrell BF., 2000. Mechanisms of eastern Mediterranean rainfall variability. JOURNAL OF

THE ATMOSPHERIC SCIENCES, Vol 57, Iss 19, pp 3219-3232

Fernandez J, Saez J, Zorita E. , 2003. Analysis of wintertime atmospheric moisture transport and

its variability over the Mediterranean basin in the NCEP-Reanalyses. Climate Research 23,

195-215.

Guillet N, Hegerl, GC, Allen MR, and Scott PA, 2000: Implications of changes in the Northern

Hemisphere circulation for the detection of anthropogenic climate change. Geophys. Res.

Lett. 27, 993-996.

Goodess CM; Jones PD, 2002. Links between circulation and changes in the characteristics of

Iberian rainfall. INTERNATIONAL JOURNAL OF CLIMATOLOGY, Vol 22 13, pp 1593-1615.

Hasanean HM, 2001. Fluctuations of surface air temperature in the Eastern Mediterranean.

THEORETICAL AND APPLIED CLIMATOLOGY, Vol 68, Iss 1-2, pp 75-87

Homar, V., C. Ramis, R. Romero, S. Alonso, J. A. García Moya, and M. Alarcón, 1999: A case of

convection development over the western Mediterranean Sea: A study through numerical

simulations. Meteorol. Atmos. Phys. 71, 169-188.

Hurrell JW., 1995. Decadal trends in the North-Atlantic Oscillation-regional temperatures and

precipitation. SCIENCE, Vol 269, Iss 5224, pp 676-679

Hurrell JW, VanLoon H.1997. Decadal variations in climate associated with the north Atlantic

oscillation. CLIMATIC CHANGE, Vol 36, Iss 3-4, pp 301-326.

Krichak SO; Kishcha P; Alpert P., 2002. Decadal trends of main Eurasian oscillations and the

Eastern Mediterranean precipitation. THEORETICAL AND APPLIED CLIMATOLOGY, Vol

72, Iss 3-4, pp 209-220.

Lamb P, Peppler R, 1987 The North Atlantic Oscillation: Concept and an application. Bull. Amer.

Meteor. Soc.68, 1218-1225

Mariotti A; Zeng N; Lau KM., 2002. Euro-Mediterranean rainfall and ENSO - a seasonally varying

relationship. GEOPHYSICAL RESEARCH LETTERS, Vol 29, Iss 12, art. no.1621.

Osborn TJ, Briffa KR, Tett SFB, Jones PD and Trigo RM, 1999. Evaluation of the North Atlantic

Oscillation as simulated by a coupled climate model. Clim. Dyn. 15, 685-702.

Pastor F; Estrela MJ; Penarrocha D; Millan MM, 2001. Torrential rains on the Spanish

Mediterranean coast: Modelling the effects of the sea surface temperature. JOURNAL OF

APPLIED METEOROLOGY, Vol 40, Iss 7, pp 1180-1195

Pozo-Vázquez D, Esteban-Parra M. J.,. Rodrigo F. , Castro-Díez Y, 2001. A study of NAO

variability and its possible non-linear influences on European surface temperature. Clim.

58

Dynamics 17, 701-715.

Rodriguez-Fonseca, Belen, de Castro, Manuel, 2002: On the connection between winter

anomalous precipitation in the Iberian Peninsula and North West Africa and the summer

subtropical Atlantic sea surface temperature. Geophys. Res. Lett., 29,

10.1029/2001GL014421.

Romero, Ramis C, Guijarro JA, 1999. Daily rainfall patterns in Spanish-Mediterranean area: an

objective classification. Int. Journal of Climatology 19, 95-112

Quadrelli R; Pavan V; Molteni F., 2001. Wintertime variability of Mediterranean precipitation and its

links with large-scale circulation anomalies. CLIMATE DYNAMICS, Vol 17, Iss 5-6, pp 457-

466

Ribera P; Garcia R; Diaz HF; Gimeno L; Hernandez E., 2000. Trends and interannual oscillations

in the main sea-level surface pressure. patterns over the Mediterranean, 1955-1990.

GEOPHYSICAL RESEARCH LETTERS, Vol 27, Iss 8, pp 1143-1146.

Rimbu N; Le Treut H; Janicot S; Boroneant C; Laurent C., 2001. Decadal precipitation variability

over Europe and its relation with surface atmospheric circulation and sea surface temperatur.

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol 127, Iss 572,

pp 315-329

Rodo X; Baert E; Comin FA., 1997. Variations in seasonal rainfall in southern Europe during the

present century: Relationships with the North Atlantic Oscillation and the El Nino Southern

Oscillation). CLIMATE DYNAMICS, Vol 13, Iss 4, pp 275-284

Saenz J; Zubillaga J; Rodriguez-Puebla C., 2001. Interannual variability of winter precipitation in

northern Iberian Peninsula. INTERNATIONAL JOURNAL OF CLIMATOLOGY, Vol 21, Iss

12, pp 1503-1513

Sumner G; Homar V; Ramis C., 2001. Precipitation seasonality in eastern and southern coastal

Spain. INTERNATIONAL JOURNAL OF CLIMATOLOGY, Vol 21, Iss 2, pp 219-247

Trigo IF; Bigg GR; Davies TD. ,2002. Climatology of cyclogenesis mechanisms in the

Mediterranean. MONTHLY WEATHER REVIEW, Vol 130, Iss 3, pp 549-569

Trigo IF; Davies TD; Bigg GR., 2000. Decline in Mediterranean rainfall caused by weakening of

Mediterranean cyclones. GEOPHYSICAL RESEARCH LETTERS, Vol 27, Iss 18, pp 2913-

2916

Trigo IF; Davies TD; Bigg GR, 1999. Objective climatology of cyclones in the Mediterranean

region. JOURNAL OF CLIMATE, Vol 12, Iss 6, pp 1685-1696.

Trigo R and Palutikof JP, 1999. Simulation of daily temperatures for climate change scenarios over

Portugal: a Neural Network Model approach". Climate Research 13, 45-59.

Ulbrich U, Christoff M, Pinto JG and Corte-Real J, 1999. Dependence of Winter Precipitation over

Portugal on Nao and baroclinic wave activity. Int. Journal of Climatology 19, 379-390

Valero F, Luna, M. Y. and M. L. Martin, 1997: An overwiew of a heavy rain event in Southeastern

59

Iberia: the role of large-scale meteorological conditions. Ann. Geophysicae, 15, 494-502.

Zorita E and Gonzalez-Rouco F., 2000 Disagreement in predictions of future behaviour of the

Arctic Oscillation as simulated in two different climate models: implications for global

warming. Geophys. Res. Lett. 27, 1755-1758

Xoplaki E; Luterbacher J; Burkard R; Patrikas I; Maheras P., 2000. Connection between the large-

scale 500 hPa geopotential height fields and precipitation over Greece during wintertime.

CLIMATE RESEARCH, Vol 14, Iss 2, pp 129-146

Xoplaki E; Gonzalez-Rouco JF; Luterbacher J; Wanner H., 2003. Mediterranean summer air

temperature variability and its connection to the large-scale atmospheric circulation and

SSTs. CLIMATE DYNAMICS, Vol 20, Iss 7-8, pp 723

60

3. Variability of the Mediterranean Sea Level and Oceanic

Circulation and their Relations to Climate Patterns

Mikis Tsimplis1

1 Southampton Oceanography Centre, Southampton UK ([email protected])

• The Mediterranean Sea is not at a steady state.

• Significant and largely unexplained changes in the water mass characteristics, sea level and

circulation have been observed.

• The observed variability consists of long-term slow changes in the deep and the upper waters

and rapid changes in the circulation.

• Large scale meteorological forcing for example the North Atlantic Oscillation, anthropogenic

influences (for example the damming of rivers) as well as changes to the hydraulic conditions

at the Strait of Gibraltar appear to be related to some of these changes.

• The variability observed is not uniform over the whole Mediterranean area but it varies at basin

wide or even smaller scales thus demanding more intensive observation and analysis.

3.1 Climatic Forcing of Mediterranean Sea Level and Oceanic

Circulation Variability

3.1.1 Background

The progressive improvement of existing climatological data sets indicates that the Mediterranean

Sea is not in a steady state and is potentially very sensitive to changes in atmospheric forcing. The

Mediterranean Sea acts as a miniature ocean when thermohaline circulation, dense water

formation and the impact of environmental forcing are concerned (Bethoux et al., 1999). The

excess evaporation over precipitation and a negative heat budget (Garrett et al., 1993; Gillman and

Garrettt, 1994) are balanced by a two layer exchange at the Strait of Gibraltar (sill depth about

280m) comprising a relatively cool, fresh (15˚C, 36.2psu) upper water inflow and a relatively warm

and saltier (13.5˚C and 38.4 psu) outflow to the Atlantic (Tsimplis and Bryden, 1999). The inflow of

Atlantic water is highly modulated by the tidal signal and the atmospheric forcing at the Strait of

Gibraltar but as it correlates well with the cross strait sea level component it is likely to be easily

61

monitored (Tsimplis and Bryden, 1999). The eastward spreading Atlantic water becomes

progressively warmer and saltier due to atmospheric forcing and to mixing with underlying water

masses (Malannotte-Rizzoli and Hect, 1988 for a review and Font et al., 1998 for recent

observations). The surface Mediterranean circulation as seen from altimetry is a complex

combination of mesoscale and large-scale variations (Larnicol et al., 1995).

3.1.2 Water Formation

The most important Mediterranean water mass is the Levantine Intermediate Water (LIW) which is

formed at the Levantine Basin while the Winter Intermediate Water (WIW) both cooler and fresher

than the LIW is formed in the western basin (Send et al., 1999). Deep water formation takes place

in the Gulf of Lions (Stommel, 1972) where heat dominated local buoyancy flux appears to

determine the depth of the deep water convection (Mertens and Schott, 1998). Deep water

formation also takes place in the North Adriatic and up to 1987 it was clear that most of the deep

water of the eastern Mediterranean was of that origin (Roether et al., 1983). Nevertheless between

1987 and 1995 water, probably formed within the Cretan Basin, filled in the deep Eastern

Mediterranean demonstrating a new source of deep water formation (Roether et al. 1995; Brankart

and Pinardi 2001).). The deep water outflow ceased after 1997 and thus the phenomenon was

termed the Eastern Mediterranean Transient (EMT). There have subsequently been signs that the

Adriatic will resume its role as the deep water formation of the Eastern Mediterranean (Klein et al.,

2000). Nevertheless it has become apparent that the deep water formation at the Eastern

Mediterranean is not a regular event, or if it one was it is not any more

The mechanism by which the EMT was triggered remains unclear. It has been suggested that a

combination of long term reduction in precipitation combined with exceptionally cold winters was

responsible for the initiation of the deep water formation (Lascaratos et al., 1999). Nevertheless

other views exist. For example Mallanotte-Rizzoli et al., (1999) attribute the initiation of the EMT to

changes in the circulation of the LIW while Zervakis et al. (2000) suggest that deep water formation

started in the North rather than the Southern Aegean and was linked with reduced outflow from the

Black Sea. Recently evidence for deep water formation at the eastern parts of the Chios Basin has

also been suggested .Moreover there is disagreement on whether the EMT is a phenomenon

which has never occurred before in observational records (Lascaratos et al., 1999) or whether it

has happened before (Theocharis et al.,1999, Josey, 2003). In addition (Josey, 2003) has shown

that the primary contribution to buoyancy loss at the ocean surface during the exceptionally cold

winters was due to severe heat loss and that reduced precipiation could only have played a minor

role.

There is further disagreement on whether all the contributory factors i.e. the gradual

preconditioning of the surface waters by reduced evaporation, the exceptionally cold winters and

62

the reduction of fresh waters from the Black Sea are connected with large scale atmospheric

forcing and in particular with the North Atlantic Oscillation. This suggestion originally made by

Tsimplis and Josey (2001) on the basis of budget estimates of E-P and river outflow may be true in

relation to the gradual preconditioning of the upper waters as it is established that cooling and

increases in salinity accompanied the increase in the positive status of the NAO during the recent

decades (Tsimplis and Rixen; 2002, Tsimplis et al., 2003). Nevertheless Josey (2003)

demonstrates that the exceptionally cold winters which are likely to be responsible for the initiation

of the EMT are uncorrelated with the NAO although anomalously high pressure over the North-

East Atlantic has been identified as the driving mechanism for the strong buoyancy loss in the

Aegean Sea at the time of the EMT In contrast Rixen et al. (2003) argue that the NAO was directly

responsible for the initiation of the EMT. Moreover Boscolo and Bryden, (2001) suggest that river

outflow reduction is likely to have played a role in the EMT. Thus there is scope in exploring the

relationship between deep water formation in the Mediterranean and the larger scale forcing

whether this is the NAO or one of the other principal components of atmospheric variability.

Moreover, because it is unclear whether there have been previous shifts in the deep water

formation mechanisms of the Mediterranean Sea, resolving whether and when they have

happened is an issue of high priority.

3.1.3 Deep Water Changes

The temperature and salinity of the deep waters of the Western Mediterranean Sea have been

increasing after 1960 (Bethoux et al., 1990; Bethoux et al., 1998; Leaman and Schott, 1991;

Rohling and Bryden, 1992, Tsimplis and Baker, 2001). Rohling and Bryden (1992) suggest that

increases have been caused by the damming of rivers and especially of the Nile while Bethoux

(1990) and Tsimplis and Baker (2001) suggest that local atmospheric forcing is responsible for

these changes. Very recently, even stronger trends were observed in the intermediate and deep

water column of the southern Tyrrhenian as deep as 3000 m but the forcing of these changes

remains unresolved. The relationship of the water mass characteristics with the circulation changes

is in most cases speculative.

3.1.4 Straits

The exchange of water at the Strait of Gibraltar is hydraulically controlled both at the Camarinall

Sill and at the Tarifa Narrows (Farmer and Armi, 1986). Because of the difficulties of obtaining

continuous measurements at the Strait no documented influence of the NAO or other climatic

parameter on the exchange has been documented. Nevertheless it would be surprising if there is

no influence. Ross et al. (2000) suggest that the Mediterranean sea level rise between 1994-1997

63

causes reduction in the inflow of Atlantic water at the Strait. Other straits within the Mediterranean

basin show variability which in some case related to NAO and sometimes not. For example the

transport in the Corsica Channel (1985-today) shows a significant seasonal and interannual

variability significantly correlated with the NAO Index. In contrast, the data available since 1993 for

the Sicily Strait, indicate that the variability of the water flux is less accentuated and independent

from the NAO Index (Astraldi et al., 1999; Vignudelli et al., 1999).

3.1.5 Sea Level

Probably the most dramatic of all the changes as far as impacts on society are concerned are the

changes in the Mediterranean sea level. . Since 1992 sea level change is monitored by satellite

altimetry, that provides a dense and complete coverage of the sea, while previously only few tide

gauge data were available, providing a discrete data coverage over several years (Woodworth,

2003). Before 1960 sea level in the Mediterranean has been increasing in line with the estimated

global value of 1.8 mm/yr (Church et all, 2001), between 1960 and the beginning of the 1990s sea

level in the Mediterranean Sea has dropped by 2-3 cm (Tsimplis and Baker, 2001) in spite of the

general atmospheric warming. Part of this reduction was due to direct atmospheric pressure

increases linked with the North Atlantic Oscillation (Tsimplis and Josey, 2001) while cooling of the

upper waters also played a part in the Aegean and the Adriatic Seas (Tsimplis and Rixen, 2002).

Notably after 1993 sea level in the Eastern Mediterranean increased rapidly with rates 10 times

that of global sea level rise (Fenoglio-Marc, 2002; Cazenave et al., 2001). These increases appear

temperature related, as it corresponds to an increase of the observed sea surface temperature

(Fenoglio-Marc 2001), and connected with the reduction of transport of Atlantic Water to the

Eastern Mediterranean as it has been diverted northwards in the Ionian by changes in the wind

stress (do you have a reference here?). Which part of this long-term sea level change is due to

steric effect has still to be quantified. Coherence analysis between mean sea level and

atmospheric pressure shows a significant departure from a standard inverse barometric, that

confirms the role of the Strait of Gibraltar and Sicily in limiting the water exchange (Le Traon and

Gauzelin 1997). Venice and the Nile delta are two of the most vulnerable areas of the

Mediterranean to sea level changes. The impact of the NAO has so far being to reduce the

damage of rising sea levels especially during the winter where most storms occur. Nevertheless

the recent fast sea level rise indicates that the recent decades have been exceptional and it is

therefore essential to monitor and understand these changes in order to assess future projections

of sea level in the region.

64

3.1.6 Modelling

Significant efforts have been made in the past years to model the circulation of the Mediterranean

Sea (Malanotte-Rizzoli et al., 1991; Pinardi and Masetti 2000) and to explore the forcing of,

particularly the EMT (See for a review Lascaratos et al., 1999; Meyers et al., 1999; Samuel et al.,

1999) and the generation of the LIW (Nittis and Lascaratos, 1998). Nevertheless in the absence of

measurements of LIW formation rates and a comprehension of the full mechanism of the initiation

of the EMT these studies are largely exploratory and depend on the tuning of the model. The

models show a large amplitude interannual variability of the circulation and water mass structure

associated with the anomalies of atmospheric forces over the basin (Pinardi et al 1997, Demirov

and Pinardi 2002). Nevertheless these models are the only consistent way of looking at the water

formation, water transport and surface forcing rates simultaneously and thus they are useful in

linking the different parts of the puzzle of the Mediterranean circulation and its variability.

3.2 Outlook and Future Challenges

• Investigation and identification of the main modes of temporal and spatial variability of

circulation in the Mediterranean and their response to different modes of atmospheric

variability.

• Analyse the relationship between changes in the Mediterranean circulation and regional

climate variability especially intermediate and deep water formation as well as paths of

transport of the water masses.

• Identify the links between climate variability in each part of the Mediterranean area and global

scale climate patterns (NAO, Monsoon, ENSO)

• Identify the links between changes in water mass parameters (e.g. temperature and salinity)

with circulation variability.

• Identify previous shifts in the circulation of the Mediterranean Sea from historical hydrographic

and climatic records.

• Identify sub-basin scale variability and link it with basin scale variability. In particular

quantification of exchange of particular water masses between sub-basins are needed.

• Quantify the freshwater contribution to the Mediterranean and its time variability and explore its

contribution to the water budget of the Mediterranean.

65

• Identify climate change related variability in the surface forcing parameters (wind stress,

temperature, precipitation etc.) and river runoff and its impact on circulation changes.

Seasonal cycles, extremes and changes in the distribution of parameters should be explored.

• Assessments of future changes in Mediterranean circulation due to enhanced man-made

greenhouse forcing, including dynamical as well as statistical downscaling techniques, referring

to particular regions and to the whole Mediterranean area as well.

• Resolve the contribution of forcing parameters on sea level variability. In particular, for long-

term changes: quantify steric and non-steric long-term sea level change using for example the

existing Atlas (WOA98 and Medar/Medatlas).> Explore the links between sea level and sub-

basin scale circulation change.

• Produce future sea level changes under scenarios of global change

• Use the available models describing the Mediterranean circulation for climate variability related

research.

3.3 References

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Theocharis and J. Tintore, The role of straits and channels in understanding the

characteristics of Mediterranean circulation. Progr. in Oceanogr., 44, 65-108, 1999.

Bethoux J.P., B. Gentili, P. Morin, E. Nicolas,C.Pierre, D.Ruiz-Pino, The Mediterranean Sea: a

miniature ocean for climatic and environmental studies and a key for the climatic functioning

of the North Atlantic. Progress in Oceanog. 44, 131-146, 1999

Bethoux, J. P., B. Gentili, J. Raunet and D.Tailliez, Warming trend in the Western Mediterranean

deep Water. Nature, 347, 660-662, 1990.

Bethoux, J.P., B. Gentili and D. Tailliez, Warming and freshwater budget change in the

Mediterranean since the 1940s, their possible relation to the greenhouse effect. Geophys.

Res. Let., 25, 1023-1026, 1998.

Boscolo R, Bryden H, Causes of long-term changes in Aegean sea deep water, Oceanologica

Acta , 24 (6): 519-527, 2001

Cazenave A., C. Cabanes, K. Dominh and S. Mangiarotti, Recent sea level changes in the

Mediterranean Sea revealed by TOPEX/POSEIDON satellite altimetry. Geophys. Res. Let.,

28(8), 1607-1610, 2001.

Church J.A., J.M. Gregory, P. Huybrechts, M. Kuhn, K. Lambeck, M.T. Nhuan, D. Qin and P.L.

Woodworth, Changes in sea level. Chapter 11 of the Intergovernmental Panel on Climate

Change Third Assessment Report, Cambridge University Press, 2001.

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Demirov E. and Pinardi N. Simulation of the Mediterranean Circulation from

1979 to 1993: Part I. The interannual variability, Journal of Marine Systems 33-34, 23-50, 2002

Farmer D.M. and L.Armi, Maximal two-layer exchange over a sill and through the combination of a

sill and a contraction with barotropic flow. J. Fluid Mech., 164, 53-76, 1986.

Fenoglio-Marc L., Long-term sea level change in the Mediterranean Sea from multi-satelitte

altimetry and tide gauges. Physics and Chemistry of the Earth, 27, 1419-1431 (2002).

Font J, Millot C, Salas J, Julia A, Chic O, The drift of modified Atlantic water from the Alboran Sea

to the eastern Mediterranean, Scientia Marina, 62 (3): 211-216, 1998.

Garrett C., R. Outbridge and K. Thompson, Interannual variability in Mediterranean heat and

buoyancy fluxes. J. of Climate, 6, 900-910, 1993

Garrett, C., The role of the Strait of Gibraltar in the evolution of Mediterranean water, properties

and circulation. Bulletin de l'Institut oceanographique, Monaco, no special 17, 1-19, 1996.

Gilman C and C . Garrett, Heat-flux parameterizations for the Mediterranean-sea - the role of

atmospheric aerosols and constraints from the water-budget. J. Geophys. Res. (Oceans), 99

(c3): 5119-5134.

Josey S.A, Changes in the heat and freshwater forcing of the eastern Mediterranean and their

influence on deep water formation. J Geophys Res. -Oceans 108 (C7): art. no. 3237, 2003

Larnicol G, LeTraon PY, Ayoub N, DeMey P, Mean sea level and surface circulation variability of

the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry. J. Geophys. Res., 100

(C12): 25163-25177,1995

Lascaratos A, Roether W, Nittis K, Klein B, Recent changes in deep water formation and spreading

in the eastern Mediterranean Sea: a review. Progress In Oceanography 44 (1-3): 5-36 1999.

Leaman, K. D. and F. A. Schott, Hydrographic structure of the convection regime in the Gulf of

Lions: Winter 1987. J. Phys. Oceanogr. 21, 575-598, 1991.

Le Traon P.-Y. and Gauzelin P., Response of the Mediterranean mean sea level to atmospheric

pressure forcing, . J. Geophys. Res. 102, C1, 973-984, 1997.

Klein, B., W. Roether, G. Civitarese, M. Gacic, B.B. Manca and M.R. d’Alcala. Is the Adriatic

returning to dominate the production of Eastern Mediterranean Deep Water? Geophys. Res.

Let., 27, 3377-3380, 2000.

Malanotte-Rizzoli P and A. Hecht, Large-scale properties of the eastern Mediterranean - a review.

Oceanol Acta 11 (4): 323-335, 1998.

Malanotte-Rizzoli P, Manca BB, d'Alcala MR, Theocharis A, Brenner S, Budillon G,

Ozsoy E, The Eastern Mediterranean in the 80s and in the 90s: the big transition in the

intermediate and deep circulations. Dynamics of Atmospheres and Oceans, 29 (2-4): 365-

395, 1999

67

Malanotte-Rizzoli P, Bergamasco A, The Wind And Thermally Driven Circulation Of The Eastern

Mediterranean-Sea .2. The Baroclinic Case, Dynamics Of Atmospheres And Oceans, 15 (3-

5): 355-419 APR 1991

Mertens C and F., Interannual variability of deep water formation in the North-western

Mediterranean Sea. J. Phys. Oceanogr.28, 1410-1424.

Myers, P. G., K. Haines and S. Josey, On the importance of the choice of wind stress forcing to the

modelling of the Mediterranean Sea circulation. J. Geophys. Res., 103, C8, 15729 - 15749,

1998.

Nittis K, Lascaratos A, Diagnostic and prognostic numerical studies of LIW formation, Journal Of

Marine Systems, 18 (1-3): 179-195 DEC 1998

Pinardi N., Korres G., Lscaratos A., Roussenov V. and Stanev E. Numerical simulation of the

interannual variability of the Mediterranean Sea upper ocean circulation, Geophys. Res.

Lett. 24, 425-428, 1997

Pinardi N. and Masetti E. Variability of the large scale general circulation of the Mediterranean Sea

from observations and modelling: a review, Palaeo 158, 153-174, 2000

Pollak, M. I., The sources of deep water of the Eastern Mediterranean Sea. Deep Sea Res., 10,

128-152, 1951.

Roether W., Schlosser P. Kuntz R., Weis W., Transient tracers studies of the thermohaline

circulation of the Mediterranean. MS for Mediterranean Circulation, NATO Workshop, edited

by H. Charnock 44pp. 1983.

Roether W., B.B. Manca, B. Klein, D. Bregant, D. Georgopoulos, V. Beitzel, V. Kovacevic and A.

Luchetta, Recent changes in Eastern Mediterranean deep waters. Science, 271, 333-335,

1996.

Rohling E. and H. Bryden, Man-induced salinity and temperature increases in the Western

Mediterranean Deep Water. J. Geophys. Res., 97, 11191-11198, 1992.

Ross., T., C. Garrett and P.-Y. Le Traon, Western Mediterranean sea-level rise: changing

exchange flow through the Strait of Gibraltar, Geophys. Res. Let., 27, 2949-2952, 2000.

Samuel, S., K. Haines, S. Josey and P.G. Myers, Response of the Mediterranean Sea

thermohaline circulation to observed changes in the winter wind stress field in the period

1980-1993. J. Geophys. Res., 104, 7771-7784, 1999.

Send U., J. Font, G. Krahmann, C. Millot, M. Rhein and J. Tintore, Recent advances in observing

the physical oceanography of the western Mediterranean Sea. Progress in Oceanogr., 44,

37-64, 1999.

Stommel H., Deep winter-time convection in the Western Mediterranean Sea. Studies in Physical

Oceanography, Vol.2 edited by A.L. Gordon, Gordon and Breach, New York, 207-218 1972

68

Theocharis A, Nittis K, Kontoyiannis K, Papageorgiou E, Balopoulos E, Climatic changes in the

Aegean Sea influence the Eastern Mediterranean thermohaline circulation (1986-1997),

Geophys. Res. Let., 26 (11): 1617-1620 JUN 1 1999

Tsimplis M.N. and T.F. Baker, Sea level drop in the Mediterranean Sea: An indicator of deep water

salinity and temperature changes? Geophys. Res. Let. , 27(12), 1731-1734, 2000.

Tsimplis, M.N., and H.L. Bryden, Estimation of transports through the Strait of Gibraltar, Deep-Sea

Res., Part I, 47, 2219-2242, 2000.

Tsimplis M.N. and S.A. Josey, Forcing of the Mediterranean Sea by atmospheric oscillations over

the North Atlantic. Geophys. Res. Let., 28(5) 803-806,2001

Tsimplis M.N. and M. Rixen, Sea level in the Mediterranean Sea: The contribution of temperature

and salinity changes. Geophys. Res. Let. 29(3) art. no. 2136,2002.

Vignudelli S., G.P. Gasparini, M. Astraldi and M.E. Schiano, A possible influence of the North

Atlantic Oscillation on the circulation of the Western Mediterranean Sea. Geophys. Research

Let., 26, 623-626, 1999.

Woodworth P.L., Some comments on the long sea level records from the Northern Mediterranean,

Journal of Coastal Research, Vol. 19, N.1, 2003

Zervakis, V., D. Georgopoulos and P. G. Drakopoulos The role of the North Aegean in triggering

the recent Eastern Mediterranean climatic changes. J. Geophys. Res. 105 (C11) 26103-

26116, 2000.

69

4. Evaluation of the Regional Variability in Connection with

Weather Patterns

P.Lionello1, S. Krichak2 and I.F. Trigo3

1 University of Lecce, Italy ([email protected])2 Tel Aviv University, Israel ([email protected])3 University of Lisbon, Portugal ([email protected])

4.1 Climatology of Cyclones in the Mediterranean

4.1.1 The Role of the Atlantic Storm Track and of the North Atlantic Oscillation

In recent years, two distinct approaches have been used to study the storm activity over the North

Atlantic and Europe: storm track algorithms and analysis of synoptic variability. Usually, storm track

algorithms are sophisticated methods that can detect the regions of storm development

(cyclogenesis) and decay (cyclolysis) as well as the specific paths of each individual storm (Murray

and Simmonds, 1991; Serreze et al., 1997; Trigo et al., 1999; Trigo et al., 2002, Lionello et al

2002). Analysis of synoptic variability is a simpler approach which corresponds to the identification

of the synoptic variability using a band-pass filter that retains mainly variability on the 2-8 day

period range of SLP or 500 hPa geopotential. This second approach has been more widely

applied to quantify the synoptic activity associated with high and low NAO index (e.g. Rogers,

1997; Ulbrich and Christoph, 1999; Trigo R. et al., 2002). However the first technique has also

been used to show the areas of significant difference in storm activity between winters with high

and low NAO index (Serreze et al. 1997). Both approaches show that the major cyclone variability

mode is strongly associated with NAO (North Atlantic Oscillation), with centres of action over the

Denmark Strait (positive NAO) and over the Bay of Biscay (negative NAO).

The Mediterranean region is only partially affected by the North Atlantic Storm track, whose main

path crosses the Northern Atlantic towards Northern Europe. Consequently, the main mode of the

North Atlantic Storm track variability, which describes its north-south shift and intensification over

the Atlantic, is only marginally related to the frequency and intensity of the cyclones in the

Mediterranean region. In fact, a clear signal of NAO storm-track dependence over the

Mediterranean has not been detected in all studies., though Trigo et al. (2000) have clearly

demonstrated that an association exists and that it depends on the cyclone structural

characteristics. The parallel analysis of the low frequency SLP (Sea Level Pressure) variability

patterns and of the frequency of cyclones in the Mediterranean region shows that this is associated

also with teleconnection patterns other than NAO. High level of SLP synoptic scale variability is

70

associated with the positive phases of the SENA pattern (Southern Europe Northern Atlantic) and,

to a minor degree, to the SCAN (SCANdinavian) and EATL (Eastern ATLantic) [Rogers 1990] and

a similar East Atlantic/Western Russia (EAWR) [Krichak et al. 2000, 2002] patterns. While NAO is

linked to the latitudinal variability of the Storm-track in the central Atlantic, instead the bulk of the

variability over Central and Southern Europe and over the Mediterranean region is linked to low

frequency patterns, whose centres of actions are localized over Europe and North-eastern Atlantic.

However, such large-scale analysis is generally based on coarse resolution fields where the sub-

synoptic and meso-scale characteristics of the cyclones in the Mediterranean region are poorly

reproduced. Moreover, usually, these teleconections are defined on a monthly scale. Naturally,

sub-monthly large scale features such as the well known, and relatively frequent, euro-Atlantic

blocking episodes (Tibaldi et al, 1997) can influence the trajectory of storm tracks and their

associated precipitation fields (Trigo R., 2004).

Note that Mediterranean features are well distinct in the global storm track structure, though their

amplitude is smaller than that of the Atlantic and Pacific centres of activity. In fact, the Northern

Hemisphere storm track presents a separate branch crossing the Mediterranean region, with areas

of cyclogenesis in the western Mediterranean and of prevalent cyclolisis in the Central and Eastern

Mediterranean (Hoskins and Hodges, 2002).

4.1.2 Sub-Areas of Cyclogenesis, Seasonality and Generation Mechanisms

The analysis of the ERA-15 (ECMWF ReAnalysis) at T106 resolution shows that the characteristic

space and time scales of cyclones in the Mediterranean region are smaller than in the Atlantic

(Trigo et al.1999). Over 65% of cyclones have a maximum radius less than 550 km, compared to

the 1000-2000 km of the Atlantic synoptic systems. If the shortest living cyclones (with duration

lower than 12h) are excluded, the average life of cyclones in the Mediterranean region is about 28

hours, compared to 3 – 3.5 days in the Atlantic. Radius and maximum gradient tend to scale with

the minimum pressure. In general cyclones are deeper and have a larger radius in the western

than in the Eastern Mediterranean. A recent evaluation (restricted to the western Mediterranean

region, Picornell et al.2001), based on higher resolution fields (computed by HIRLAM at 0.5°

resolution), and including also short living cyclones, produced smaller space and time scale values.

In this data-set the radius of most cyclones is in the range from 150 to 350km (the mean value is

255km) and the most frequent lifetime of intense cyclones is about 18-24hours. Positive deepening

values are mostly lower than 2hPa (6h)-1, though value as high as 10hPa(6h)-1 can be observed.

Therefore, also deepening rates are smaller than in the Atlantic. Moreover, many cyclones in the

Mediterranean region have null or even negative deepening rate, meaning that they are originated

in neighbouring regions and cross the Mediterranean during their attenuation phase.

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The cyclogenesis within the Mediterranean region is characterized by a rich mesoscale structure,

with many different generation areas (tab.1), cyclones types, generation mechanisms and

seasonal variability (Alpert et al 1990a, Alpert et al 1990b, Trigo et al 1999, Maheras et al 2001,

Picornell et al.2001).

AREA SEASONALITY RADIUS (km)Sahara Spring , Summer 530-590Gulf of Genoa Whole Year 530-380Southern Italy Winter 520Cyprus Spring, Summer 330-460Middle East Spring, Summer 320-460Aegean Sea Winter, Spring 500Black Sea Whole year 380-400Iberian Peninsula Summer 410

Table 1. Cyclogenetic region in the Mediterranean area (after Trigo et al. 1999). Values represent

average values in selected months (January, April, August)

Cyclogenesis occurs in the Gulf of Genoa, Spain, Southern Italy, Northern Africa, Aegean Sea,

Black Sea, Cyprus, Middle East (Figure 8). Despite the use of different methodologies, selection

criteria and data sets, these studies agree on the spatial location of cyclone generation (but for the

Aegean Sea, which is present only in Trigo et al.1999). Unfortunately, the use of so many different

pre-requisites undermines the possibility of comparing results between these papers. Recent

studies, based on automated database methods (Campins et al 2000), have identified even

smaller meso-scale structures and identified in the western Mediterranean 7 types of cyclones, on

the basis of shape and intensity of the associated circulation.

Figure 8. Number of cyclogenesis events detected per 2.25o X 2.25o in January, April, and August

from 1979 to 1996 in ECMWF re-analyses (from Trigo et al. 1999)

The analysis of the monthly fields favours the definition of three seasons: winter, spring and

summer, while autumn appears as a transitional period with large inter-annual variability, whose

72

months could be characterized as late summer or early winter. However, seasonal characterization

is not fully adequate, because of large inter-monthly variability (Alpert et al 1990a).

The role of orographic cyclogenesis (Buzzi and Tibaldi 1978) is fundamental in the Gulf of Genoa,

in the North-western Mediterranean Sea, and in the Aegean Sea. In general areas with high

concentration of cyclones are located near mountain ranges (mostly south of the Pyrenees, the

Apennines and the Alps). Cyclones that develop over the three most active areas in winter are

essentially subsynoptic lows, which may occur consecutively, being triggered by passage of a

major system over northern Europe, whose path is a main factor determining their generation

(Trigo et al. 2002). Thermal lows are characteristic of the Iberian Peninsula, and of most of the

Mediterranean basin during summer (Picornell et al, 2001, Maheras et al.2001), when the number

of low-pressure systems presents a large dependence on the daily cycle. The intensity of the

cyclogenetic activity in the south-eastern Mediterranean region is to a large extent controlled by the

large-scale synoptic processes over Europe and especially by those characterized by mid- and

upper-tropospheric southward air-mass intrusions and tropopause folding effects (Krichak and

Alpert, 2003). The processes are often associated with the formation of three-dimensional potential

vorticity structures, jet streaks and low level jets conditions over the region to the south of Alps

(Buzzi and Foschini, 2001; Liniger and Davies, 2003). Finally, Mediterranean cyclones with

hurricane-like structure, whose intensity depends critically of the latent heat release at the sea

surface have been identified, but their frequency and space-time distribution has not been

investigated, yet (Pytharoulis et al 1999).

4.1.3 Present Trends and Future Scenarios

A counting of cyclone centres (without any differentiation on intensity), based on the NCEP re-

analysis, shows a reduction of the number of cyclones in western Mediterranean and an increase

in the East (Maheras et al., 2001). Linear fit to the data implies a 10% increase/decrease. Changes

are not seasonally homogeneous. If only the rainy period (October to March) is considered, a

reduction of the number of cyclones is present also in the Eastern Mediterranean. Other studies

suggest a distinction between the increasing trend of weak cyclones and a decreasing trend of

strong cyclones in the Western Mediterranean Sea (Trigo et al. 2000). It results that the positive

trend identified in the Northern Hemisphere Storm track for the last decade of the 20th century

(Chang and Fu, 2002) is not valid in the Mediterranean region.

It is difficult to analyze Mediterranean cyclones in scenario simulations because of their intrinsic

small scale. However, the differences in cyclonic activity have been estimated for two 30-year long

slice experiments, carried out with the ECHAM-4 model at T106 resolution, simulating the present

73

and the doubled CO2 scenarios (Lionello et al 2001). The present climate is characterized by a

slightly, but statistically significant, higher overall number of cyclones (fig.2, left panel). The

doubled CO2 simulation is characterized by more extreme weather events, but the difference

between the two scenarios is hardly significant (fig.2, right panel). No variation of the regions of

formation of the cyclones has been clearly identified in this study.

Fig. 2 Left panel: cumulated distribution, that is the average number of cyclones per year (y-axis)

exceeding a given threshold (x-axis) for the present scenario (dotted line), doubled CO2 (dashed

line), and ERA-15 (solid line) scenarios. Right panels: the tails of the cumulated distributions, that

is cyclones with depth exceeding 35 hPa.(from Lionello et al.2001)

4.2 Weather Patterns and Mediterranean Environment

Obviously weather patterns have a deep influence on important environmental variables and,

particularly, on the timing and magnitude of extreme values. Cyclones are associated with rain,

wind waves and surges in shallow water areas (namely in the Gulf of Venice). Intensity of the

cyclogenetic activity in the region to a large extent is controlled by the large-scale synoptic

processes over Europe and especially by those characterized by mid- and upper-tropospheric

southward air-mass intrusions and tropopause folding effects [Krichak and Alpert, 2003]. The

processes are often associated with the formation of three-dimensional PV structures (PV

streamers), jet streaks and low level jets conditions over the region to the south of Alps [Buzzi and

Foschini, 2001; Liniger and Davies, 2003]. The conditions tend to stimulate development of the

mesoscale convective complexes and Mediterranean cyclones. Intensity, location, duration and

orientation of the systems as well as their inter-decadal trends in association of those of the main

European teleconnection patterns appear to be important elements of the eastern Mediterranean

weather and climate trends.

74

4.2.1 Precipitation

Cyclones play an important role in establishing the precipitation patterns and their variability, as

synoptic scale disturbances have been found responsible for most of the floods both in the

Northern Mediterranean and in the Middle East. Only a minority of local flush floods has been

associated to intense small convective cells, whose presence in not clear in the standard

meteorological analysis. A study carried out for the Negev Desert identified 4 classes of synoptic

disturbances responsible for most of the floods, the two more important denoted as Syrian Low

and Red Sea Trough (Kahana et al 2002). During the cool season, precipitation in the southern

part of the eastern Mediterranean (EM) region is mainly associated with cyclonic systems of the

Mediterranean origin. Major floods over Northern Italy have been mostly associated to well

developed cyclones. It is likely that this holds also for other regions.

Figure 10. Maps of the lagged correlation with the value in the point indicated by the dot for the

500hpa synoptic scale filtered geopotential height. This point is selected as a maximum of

explained variance of the precipitation field associated to the EOF describing a moisture transport

from western to eastern Mediterranean areas. The maps are meant to describe of the evolution of

the cyclonic disturbances associated with such transport (From Fernandez et al. 2002)

75

Cyclones have been found to play a key role in the internal redistribution of moisture in the

Mediterranean region (Fernandez et al. 2003). A main mode of variability describes the transport of

moisture from the Western to the Eastern Mediterranean and it corresponds to the generation (or

intensification) of cyclones in the western Mediterranean and their following eastward motion

(fig.3). Air sea interaction and a large latent heat flux are likely to play an important role in this

process.

Figure 11. Time series, and respective linear trends, of the total amount of precipitation in the

Northern Mediterranean Basin (bold curve, left axis), the total occurrence of intense Mediterranean

cyclones (light curve, right axis), and of non intense cyclones (dotted curve, right axis) for the

October to March period (from Trigo and Davies 2000).

The decrease of total precipitation during the wet season in the Northern Mediterranean has been

associated with the reduction of intense cyclones (fig.4) and the Northward shift of the storm track

over Europe in the period from the 1979 onwards (Trigo et al.2000). However, the analysis of

precipitation shows different trends depending on the intensity of the events (Alpert et al.2002).

The torrential rainfall in Italy exceeding 128 mm/day has increased percentage-wise by a factor of

4 during 1951–1995. In Spain, extreme categories at both tails of the distribution (light: 0-4 mm/d

and heavy/torrential: 64 mm/d and up) increased significantly. No significant trends were found in

Israel and Cyprus. The consequent redistribution of the daily rainfall categories-torrential/heavy

against the moderate/light intensities - is of utmost interest particularly in the semi-arid sub-tropical

regions for purposes of water management, soil erosion and flash floods impacts. Specific isolated

regions exhibit an increase of extreme rainfall in spite of the reduction of the total precipitation.

4.2.2 Waves and Surges

The surface wind, which is associated to a cyclone, produces ocean waves and, in shallow water,

storm surges. The variability of the cyclones regime has, therefore, an impact on wave field and

surge level, which can both, in turn, be considered a “proxy” of the cyclones characteristics.

76

The surge in the Northern Adriatic, and the consequent floods of Venice, is associated with intense

cyclones in the Northwestern Mediterranean. The increase of the local relative sea level is the

main cause for the increased frequency of floods. The residual variability is mainly associated to

that of the meteorological forcing. There is an indication that the frequency of moderate surges is

increasing (Pirazzoli and Tomasin, 1999) and at the same time major independent surge events do

not show any large variation (Trigo and Davies, 2002). A weak decreasing trend has been

identified in the value of extremes levels in Trieste (Raicich, 2003). This behaviour has been shown

to be consistent with the variation of the meteorological forcing in the Northern Adriatic area, that is

with more frequent moderate storms and less frequent intense storms. Scenario evaluations have

been carried out applying a downscaling procedure to T106 global simulation (Lionello et al 2002).

The comparison between the present and future climate simulations shows no statistically

significant change in the extreme surge level. Unfortunately, though greatly compensating for the

coarse resolution of the global forcing fields, this study still presents a systematic under-evaluation

of the surge extremes, which could affect the results of the comparison.

Little is known about wind waves as present climate variability and trends are concerned. The

same downscaling procedure used for the surge in the Adriatic Sea has been applied also to the

wave fields in the Adriatic Sea, and results show a reduction of the extreme wave height in a

doubled CO2 climate scenario (Lionello et al 2002).

4.3 Outlook: Crucial Topics for Ongoing Research

Though many interesting research lines have been carried out and they provide a well-established

understanding of many aspects of the Mediterranean meteorology, further research is

recommended for reconsidering these phenomena from a climate perspective.

The investigation has to assess whether the present archives provide an adequate database for

the analysis and to identify the actions required to fill gaps. On the centennial time scales it

appears that there is an unbalance between regions and phenomena that are well documented

(e.g. the surge of Venice, Camuffo 1993) and other where data are scarce and reconstruction

necessarily indirect (e.g. precipitation patterns and extremes on the whole African coast).

Moreover, for the recent decades, when meteorological observations are worldwide available and

model re-analysis have been carried out (e.g. NCEP and ERA-40 re-analysis), it has to be fully

investigated whether the sub-synoptic and mesoscale characteristics of cyclones in the

Mediterranean region are well represented in the available data archives. On this respect the

77

development of extensive, high-resolution sets of data appears extremely important for addressing

unresolved scientific issues.

4.3.1 Identification of Generation Mechanisms and Relevant Processes

The available classifications of the cyclones often lack an analysis of the generation mechanisms,

whose identification might help predicting future scenarios and the change in the frequency and

intensity of some specific cyclone types (e.g. the so-called Mediterranean lows). The availability of

regional high-resolution re-analysis, where cyclone structures are well reproduced, might be crucial

for this task. Moreover, it is important to identify the deficiencies in the models that account for

inadequacies in simulating the cyclones in the Mediterranean region and evaluating their variability.

4.3.2 Connection to Large Scale Patterns

Though there is an extensive description of the mesoscale distribution of the cyclogenesis areas

and of the cyclone tracks characteristics, including their seasonality and trends, the connection

between the climate of the cyclones in the Mediterranean region and the low frequency large-scale

variability is poorly understood. Intensity, location, duration and orientation of the systems as well

as their inter-decadal trends should be analyzed in association of those of the main European

teleconnection patterns. The importance of regional scale processes (e.g. latent heat release over

the sea) and of their variability with respect to forcing from large-scale patterns (e.g. the meridional

shift and/or intensification of the storm track over northern Europe) has not been precisely

quantified.

4.3.3 Analysis of Weather Regimes in Future Climate

The analysis of Mediterranean cyclones in future climate scenario is still preliminary and it is

undermined by the lack of adequate resolution of most simulations and the consequent doubt that

the mesoscale structure responsible for the cyclones behaviour are properly represented.

Therefore, the assessment of the role of resolution is crucial. The prediction of future scenario

would greatly benefits from an improved understanding of the teleconnection to large scale

patterns, whose behaviour in future climate has already been investigated and that are likely to be

more adequately described in global climate simulations. There is, possibly to a larger extent than

in other areas of the globe, the need to restrict uncertainty of future scenarios, and to reach a

resolution to describe mesoscale storms. It is important to understand if the absence of large

trends, resulting from the available studies, is real or it is the result of poor description of sub-

synoptic and mesoscale structures.

78

4.3.4 Impact of Variations of Weather Regimes on Precipitation Patterns, Surges,

Waves

It may be suggested that the variations observed during the last decade of the precipitation over

the Mediterranean region were associated with relatively small variations in the characteristics of

the air mass and transport. Such changes might be small and not always easily detectable,

however, their impact on local climate and climate variability is likely to be large. These variations

could have serious consequences for the rain intensities in many Mediterranean areas. The danger

is twofold. There are, on one hand, areas already under stress because of recurrent water

shortage during summer, and, on the other hand, areas where torrential rains have produced large

damages to properties and also human casualties. It is important to identify the factors responsible

for the increased of rainfall extremes and the reduction of total precipitation. It is also important to

understand the relation between intensity of precipitation and that of the cyclones, represented by

the depth of the low-pressure system or the vorticity in the area surrounding its centre.

The Mediterranean Sea is characterized by complicated coastline, with highly vulnerable areas

(e.g. the Nile’s delta and the Gulf of Venice). Though the main danger for these areas is due to the

increased coastal erosion and land losses that would be produced by sea level rise, the change in

storminess is also potentially critical. Variations of the frequency or intensity of sea storms could

further increase risks and damages. The analysis of changing wind waves and surge regimes

requires detailed impact studies carried out on the basis of sufficiently precise forcing fields, that is

it relies on accurate surface wind fields or adequate downscaling techniques.

4.4 References

Alpert P., B.U. Neeman and Y. Shay-El, 1990: Climatological Analysis of Mediterranean cyclones

using ECMWF data, Tellus, 42A, 65-77

Alpert, P., B.U.Neeman, and Y.Shay-El, 1990: Intermonthly variability of cyclone tracks in the

Mediterranean. Journal of Climate, 3, 1474-1478.

Alpert P., Ben-Gai T., Baharad A., Benjamini Y., Yekutieli D., Colacino M., Diodato L., Ramis C.,

Homar V., Romero R., Michaelides S. and Manes 2003a: The paradoxical increase of

Mediterranean extreme daily rainfall in spite of decrease in total values". Geophys. Res.

Lett., 29, 11, 31-1 - 31-4.

Buzzi A, and S. Tibaldi. 1978: Cyclogenesis on the lee of Alps: a case study, Quarterly Journal of

the Royal Meteorological Society, 104, 171-287

79

Buzzi, A., L Foschini, 2000: Mesoscale Meteorological Features Associated with heavy

precipitation in the Southern Alpine Region, Meteorol. Atmosph. Phys, 72, No. 2-4, 0131-

0146.

Campins, J., A. Genovés, A. Jansà, J. A. Guijarro, C. Ramis: A catalogue and a classification of

surface cyclones for the Western Mediterranean, Int. J. Climatol., 20, 969-984

Chang, E.K.M., and Y.Fu, 2002: Interdecadal variation in Northern Hemisphere winter storm track

intensity. Journal of Climate, 15, 642-658.

Fernandez J, Jon Saenz and Eduardo Zorita. Analysis of wintertime atmospheric moisture

transport and its variability over the Mediterranean basin in the NCEP-Reanalyses. Climate

Research 23, 195-215 (2003).

Hoskins B.J. and K.I.Hodges, 2002: New perspectives on the Northern Hemisphere winter storm

track, J.Atmos.Sci. , 59, 1041-1061

Jansà A., A. Genovés, M.A. Picornell, J. Campins, R. Riosalido, O. Carretero: Western

Mediterranean cyclones and heavy rain. Part 2: Statistical approach, Meteorol. Appl., 8, 43-

56

Kahana, R., B. Ziv, Y. Enzel, U. Dayan, 2002: Synoptic climatology of major floods in the Negev

Desert, Israel, Int. J. Climatol., 22, 822-867.

Krichak, S.O., M. Tsidulko and P. Alpert, 2000: Monthly Synoptic Patterns Associated with Wet/Dry

Eastern Mediterranean Conditions. Theor. Appl. Climatol., 65, 215-229.

Krichak S.O., P. Kishcha and P. Alpert, 2002: Decadal Trends of Main Eurasian Oscillations and

the Mediterranean Precipitation, Theor. Appl. Climatol., 72, 209-220

Krichak, S.O., P. Alpert, 2003: Low-Tropospheric Circulation Typical for Period with Eastern

Mediterranean Precipitation, Int. J. Climatol. Submitted.

Liniger, M.A. and H. C. Davies, 2003: Substructure of a MAP streamer, Quart. J. R. Meteor. Soc.

129, 633-651.

Lionello P, F.Dalan, E.Elvini ,2002: Cyclones in the Mediterranean Region: the present and the

doubled CO2 climate scenarios, Clim. Res., 22, 147-159

Lionello P., E.Elvini, A.Nizzero (2003): Ocean waves and storm surges in the Adriatic Sea:

intercomparison between the present and doubled CO2 climate scenarios, Clim. Research.,

23, 217-231

Maheras, P., H. Flocas, I. Patrikas and Ch. Anagnostopoulou, 2001: A 40 year objective

climatology of surface cyclones in the Mediterranean region: Spatial and temporal

distribution', International Journal of Climatology, 21, 109-130

Murray RJ, Simmonds I 1991: A numerical scheme for tracking cyclones centres from digital data.

Part I: Development and operation of the scheme. Aust Meteor Mag 39: 155-166.

80

Picornell M.A., A. Jansà, A. Genovés, J. Campins: Automated database of mesocyclones from the

Hirlam(INM)-0.5º analyses in the western Mediterranean, Int. J. Climatol., 21, 335-354

Pirazzoli and A.Tomasin 1999: Evoluzione delle cause recenti dell’Aqua Alta. Atti Istituto Veneto

Scienze Lettere ed Arti, CLVII, 317-344

Pytharoulis I., G.C. Craig and S.P. Ballard, 1999: Study of the Hurricane-like Mediterranean

cyclone of January 1995}, Phys. Chem. Earth (B), 24, 627-633

Rogers, J.C., 1997: North Atlantic storm track variability and its association to the North Atlantic

Oscillation and climate variability of the Northern Europe. Journal of Climate, 10, 1635-1647.

Rogers, J.C., 1990: Patterns of low-frequency monthly sea level pressure variability (1899-1986)

and associated wave cyclone frequencies. Journal of Climate, 3, 1364-1379.

Serreze MC, Carse F, Barry RG, Rogers JC 1997: Icelandic Low cyclone activity: climatological

features, linkages with the NAO, and relationships with recent changes in the Northern

Hemisphere circulation. J Clim 10: 453-464

Tibaldi S, D’Andrea F, Tosi E, Roeckner E 1997: Climatology of Northern Hemisphere blocking in

the ECHAM model. Clim Dyn 13: 649-66

Trigo, I.F., Davies, T.D. and Bigg, G.R., 1999: Objective climatology of cyclones in the

Mediterranean region, Journal of Climate 12 1685-1696.

Trigo, I.F., Davies, T.D. and Bigg, G.R., 2000: Decline in Mediterranean rainfall caused by

weakening of Mediterranean cyclones, Geophysical Research Letters 27 2913-2916.

Trigo .I.F. and T.D.Davies meteorological conditions associated with sea surges in venice: a 40

year climatology, Int.J.Climatol. 22, 787-803

Trigo, I.F, G.R.Bigg and T.D.Davies 2002: Climatology of cyclogenesys mechanisms in the

Mediterranean, Mon.Wea.Rev., 130, 549-649

Trigo RM, Osborn TJ, Corte-Real JM 2002: The North Atlantic Oscillation influence on Europe:

climate impacts and associated physical mechanisms. Clim Res 20: 9-17

Trigo RM, Trigo I.F., DaCamara C., Osborn TJ 2004: Climate impact of the European winter

blocking episodes from the NCEP/NCAR Reanalyses. Clim Dyn (in press)

Ulbrich U, Christoph M 1999: A shift in the NAO and increasing storm track activity over Europe

due to anthropogenic greenhouse gas. Clim Dyn 15:551-559.

81

5. Role of the Mediterranean Salty Water Tongue Outflowing

from Gibraltar on the North Atlantic Deep Water Cell and on

the Transitions from Glacial to Inter-Glacial Scenarios

Vincenzo Artale1, Sandro Calmanti1, Volfango Rupolo1, Alfonso Sutera2

1ENEA – Ente Nazionale per le Nuove Tecnologie, l’Energia, l’Ambiente. Roma, Italy.([email protected], [email protected], [email protected])2Università di Roma “La Sapienza”, Italy ( [email protected])

Substantial uncertainties exist, concerning the quantitative and qualitative characterization of the

main pathways for the spreading of the Mediterranean salinity anomaly in the North Atlantic.

Advances in the knowledge of the Mediterranean Outflow in the major climatic transitions are

expected from:

• investigation of large scale phenomena connected with water mass formation processes in the

North Atlantic, by means of simplified (low-dimensional) models;

• direct (lagrangian) observation of the spreading of the Mediterranean outflow;

• better representation of the exchange at the Strait of Gibraltar in large scale climatic models

(A/OGCM’s).

5.1 Introduction

The Mediterranean Sea is considered an exceptional marginal basin because all of the

fundamental oceanic processes occur in it. The interactions between the water masses originate

about 1 Sverdrup of relatively warm and saline water, flowing out of the Strait of Gibraltar. The

same contribution to the average salinity of the world ocean would be achieved by distributing over

the North Atlantic (north of 20°S) the net evaporation observed over the Mediterranean Sea. This is

equivalent to a negative contribution of about 50 mm/y to the estimated 250 mm/y fresh-water input

in the North Atlantic (Wijfels et al. 1992). Then, the contribution of the Mediterranean Outflow

Water (MOW) amounts to about 20% of the total freshwater budget over the North Atlantic (Gerdes

et al., 1999).

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5.2 State of the Art

The MOW is neutrally buoyant in the North Atlantic at about 1000m depth. Two main paths are

observed for the spreading of MOW in the North Atlantic at intermediate depth.

One is the Westward Tongue (WT), extending from the Gulf of Cadiz towards the central North

Atlantic at mid-latitudes. This path is characterized by the presence of stable subsurface eddies

containing Mediterranean water, so called meddies, that are advected by the large scale wind

driven circulation (Nof, 1982; Hogg and Stommel, 1990) and play the role of heat and salt sources

in the Mid-North Atlantic.

Another path is the Northward Undercurrent (NU), off the Iberian Peninsula, driven by geostrophic

balance of the ocean interior and mixing with the surrounding water masses (Spall, 1999).

A quantitative description of the two branches of flow is given by Sparrow et al. (2002) who

analysed recent float data and found, north of 36°N, a background northward flow, with average

velocity of 1.8±0.6 cm/s, and peaks of 10.1±3.7 cm/s off the coast of Portugal. South of 36°N they

found a weaker background flow (about 0.12 cm/s), and a strong mesoscale activity with peak

energy as high as 89 cm2/s.

5.2.1 The Signature of the Mediterranean Outflow Water (MOW) in the North Atlantic

Early documentation of the westward propagation of the Mediterranean Outflow can be found in

Wüst (1935), while Needler and Heath (1975) used the spreading of the salinity anomaly to

estimate the vertical and horizontal diffusion coefficients in the ocean. Although the strong

mesoscale activity of the WT has been investigated in great details (McDowell and Rossby, 1978;

Armi and Zenk, 1984; Bower et al., 1997), very different estimates of the role and relative

contribution of meddies to the total westward transport of salt exist. Richardson et al. (1989)

suggest a contribution of about 25% of the total transport of MOW based on a statistics of

observed meddies; Arhan et al. (1994) estimates 55% contribution of meddies; Mazè at al. (1997)

attribute to the detachment of meddies from the slope undercurrent almost all of the westward

advection of the salinity anomaly.

The NU path has also been the object of much attention in the past but is still very uncertain. Two

similar cruises, Bord-Est 2 and Bord-Est 3, conducted in 1988 and 1989, analysed by Mazè al.

(1997) and Arhan at al. (1998), produced quite different pictures of the Eastern North Atlantic. In

the first case (Bord-Est 2), a southward surface flow of 2-3 Sv off the coasts of Portugal was

inferred from hydrographical sections. Similar results are reported by Krauss and Käse (1984),

Roether and Fuchs (1988) and Paillet & Mercer (1997). In the second case (Bord-Est 3) a total

northward transport of 9-12 Sv was observed, with a well defined maximum in the MOW layer. A

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northward flow off the coast of Portugal was also observed by Haynes and Barton (1990) and by

Rios at al. (1992).

Many different factors, such as the interactions with regional topographic features, the

superposition of water masses, the strong mesoscale activity and even the seasonality of the

Azores High may seriously affect all indirect measurements of both the WT and NU paths in this

region.

Further north, the MOW can be clearly tracked at 46° N in the central Bay of Biscay (van Aken,

2000) and some authors (Reid, 1979) claim a direct contribution of the Mediterranean Sea outflow

to the Greenland-Iceland-Norwegian (GIN) Sea. Iorga and Lozier (1999) also support the so called

deep source hypothesis, in whereby a substantial northward transport of MOW, way through the

Rockall Channel, may affect directly the preconditioning in the GIN Sea.

More recently, this possibility has been severely criticized on the base of different hydrographical

observations by McCartney and Mauritzen (2001) who strongly support the alternative shallow-

source hypothesis. In this case the warm, salty inflow in the Nordic Sea is attributed to the

northward branching of the North Atlantic Current.

Bower et al. (2002) have recently attempted a direct measurement of the mid-depth circulation of

the eastern North Atlantic. They shed new light on the NU pathway in the North Atlantic observing

that the MOW is diverted into the ocean interior from the Bay of Biscay and ‘probably penetrates

north of 52°N by mixing with the North Atlantic Current in the complex area southwest of the

Porcupine Bank’. In this new scenario, relatively little water of Mediterranean origin is found inside

the Rockall Through either at the surface or at depth. Rather, almost all of the North Atlantic

Current crosses the ridge and flows into the Icelandic Sea.

To summarize, different pictures of the spreading of MOW in the North Atlantic have been

constructed from the observations and no definitive agreement has been reached yet about any of

them. Most importantly, very little is known about the variability of these pathways.

5.2.2 Modelling of MOW

Much effort as been devoted to the understanding of the mechanisms that contribute to the

shaping of the Mediterranean Outflow. Motivated by the observations of Arhan (1987), Tziperman

(1987) used a multi-layered geostrophic model to investigate the mechanism for the northward

spreading and narrowing of the Mediterranean outflow, which is attributed to the westward

propagation of Rossby waves. Spall (1994) analyses the linear instability of the meridional flow in

the eastern North Atlantic, and found that baroclinic instability could be responsible for a significant

fraction of the westward transport.

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The role of ventilation has been considered by Stephens and Marshall (1999) who stressed the

crucial role of the wind forcing in shaping the salinity tongue in the central Atlantic, while meddies

were described as a distributed source of salt in the interior of the ocean.

One of the most serious limitations to the description of the dynamics of the MOW in ocean models

is the existence of a shallow and narrow communication between the North Atlantic and the

Mediterranean Sea at the Strait of Gibraltar. A common approach to overcome this limitation is to

restore values of temperature and salinity at the eastern boundary to climatology (Stanev, 1992;

Hecht et al., 1998; Rahmstorf, 1998; New at al. 2001).

However Gerdes et al. (1999) found that realistic WT and NU can be obtained only by the

imposition of an actual inflow at the eastern boundary.

Furthermore, recent numerical simulations (Sannino et al., 2002; Sannino et al., 2004) have shown

that tides contribute up to 30% of the total exchange at the strait. They also showed that the depth

of the outflow is substantially affected by changes in its salt content, thereby suggesting that the

dynamics of the Strait of Gibraltar deserves a careful treatment especially in the case of large scale

climatic models where the resolution is usually coarse.

5.2.3 MOW and Climate

Since the Atlantic Ocean contributes for a substantial part of the poleward heat transport from the

tropics, all major climatic shifts on all ranges of time scales have been related to major changes in

the overturning circulation of the North Atlantic. On centennial time scales, the onset of different

convective regimes in the North Atlantic have been observed and related to the effect of

atmospheric activity (Root et al. 1999), although no coupling mechanism has been proposed yet.

Figure 12. Enhancement of the advective feedback in a simplified model of the thermohaline

circulation. The panels show the differences between two similar overturning regimes, which differ

by the presence of an intermediate depth anomaly. Mixed boundary conditions are employed. The

formation of strong temperature gradients at the surface and the advection of the salinity front, help

destabilizing the water column in the deep-water formation region, thereby enhancing the

circulation (from Artale et al., 2002).

85

On palaeoclimatic time scales, both observational and modelling evidences exist, supporting the

hypothesis that the transition from the Last Glacial Maximum (LGM) to the Holocene has been

related to a major reorganization of the overturning circulation of the North Atlantic (Boyle et al.,

1987; Sarnthein et al., 1994; Seidov et al., 1997).

Moreover, modelling studies suggest that under glacial conditions the meridional overturning

circulation may be characterized by different stability properties than under present day conditions

(Ganopolski et al., 1999).

Usually, two fundamental processes, which are also opposed to each other, are called for a

leading role in the instability and in the rapid transitions between different modes of operation of

the overturning circulation, namely the advective and convective feedbacks (Rahmstorf et al.,

1996). The former is related to the lateral advection of warm and salty waters towards areas of

deep-water formation. The excess of heat is rapidly lost through enhanced exchanges with the

atmosphere while the salt content is retained. This way, the instability of the water column is

enhanced and, by mass continuity, a positive feedback on the circulation is created. The

convective feedback relies on similar exchange mechanism but operates in the vertical during the

onset of deep-convection, and is related the localization of convective sites (Lenderink and

Haarsma, 1994). Tziperman (2000) illustrates how this two mechanisms may interact and generate

the instability of a given circulation regime and the formation of a new equilibrium.

Surprisingly, although the Mediterranean Outflow gives a substantial contribution to the

thermohaline forcing of the North Atlantic and thereby to the efficiency of such mechanisms, it is

usually neglected in the formulation of palaeoclimatic variability scenarios.

Nevertheless an impact of the MOW on the strength of the overturning circulation of the North

Atlantic was observed by Rahmstorf (1998). Hecht et al. (1997) achieved a stable overturning

circulation only when a realistic MOW was present. Recently Artale et al. (2002) found a

substantial impact of intermediate depth salinity anomalies on the processes affecting the stability

of meridional circulation patterns (figure 12). Furthermore, palaeoclimatic data show that the

Mediterranean Outflow was deeper (1600-2000 m), saltier (2 psu) and colder (5°C) during the Last

Glacial Maximum (Schoenfeld and Zahn, 2000). If such an outflow would have a different impact

on the stability of the MOC is still poorly understood.

5.3 Scientific Challenges

Very little is known about the role of the Mediterranean salty water tongue on the transitions from

glacial to inter-glacial scenarios. To this aim, substantial efforts must be undertaken for a

86

systematic investigation of the role of MOW on the stability and variability of the North Atlantic

overturning circulation.

5.3.1 Fundamentals

Important contributions to a better definition of the role of MOW on the efficiency of fundamental

processes such as the advective-convective feedback may come from the analysis of simplified

models, from box-models to GCM’s with idealized geometry, which have proven to be invaluable

tools for the identification of large scale oceanic processes observed in nature.

However, an unavoidable step is a deeper comprehension of the role of MOW in present day’s

climate. Important uncertainties have been stressed here, concerning the knowledge of the main

pathways for the spreading of MOW.

5.3.2 Paths of MOW

As to the WT path, one of the most challenging tasks in climate modelling on very long time scale

is to account for the role of meddies. It appears that the only possibility is to impose them as an

additional source of salt for the interior North Atlantic. However, their quantitative contribution is

still very uncertain. Therefore, a deeper understanding and modelling of the processes of meddy

formation as function of the characteristics of the Mediterranean Outflow is required. Possibly, this

leads to the construction of meddy statistics to be used in climate models.

From the experimental point of view the observation of meddies from combined satellite and in-situ

measurements (Oliveira, 2000) appears to be a promising technique for the assessment of their

role in the westward transport.

Furthermore, a robust picture of the NU path in the North Atlantic, and most importantly of its

variability, has not been established yet. This task can be achieved only by the coordination and

integration of observational and modelling activities. In particular, direct current measurement

employing float data is to be strongly supported.

It is worth noting that the modelling of the NU appears to rely on a proper description of the outflow

in large scale ocean models, which appears to be still further to come. In this context, important

information may also come from the detailed modelling of the functioning of the Strait of Gibraltar

under different climatic conditions.

87

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1187-1208.

Arhan M., Colin de Verdiere A., Memery L., 1994: The eastern boundary of the subtropical North

Atlantic. J. Phys. Oceanogr., 24, 1295-1316.

Armi L., Zenk W., 1984: Large lenses of highly saline Mediterranean Water. J. Phys. Oceanogr.,

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Artale V., S. Calmanti and A. Sutera, 2002: North Atlantic THC sensitivity to intermediate level

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Zhang H. M., 2002: Directly measured mid depth circulation in the north-eastern North

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R. Dickson, J. Lazier, J. Meincke, P. Rhines, and J. Swift, 1996: Long-term coordinated changes in

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6. Modelling of the Mediterranean regional climate

Laurent Li1, Michel Crépon2, Stefan Hagemann3, Daniela Jacob3, Richard Jones4, Shimon

Krichak5, Piero Lionello6, Annarita Mariotti7, Samuel Somot8

1 Laboratory of Dynamical Meteorology CNRS, Paris France ([email protected])2 LODYC, Paris France ([email protected])3 MPI, Hamburg Germany ([email protected], [email protected])4 Hadley Center for Climate Prediction, Exeter UK ([email protected])5 Tel Aviv University, Israel ([email protected])6 University of Lecce, Italy ([email protected])7 ENEA, Italy ([email protected])8 Meteo-France, Toulouse France ([email protected])

6.1 Scientific background

It is generally agreed that global coupled ocean-atmosphere general circulation models (CGCMs)

are the best tools to predict climate variations at seasonal and interannual scales, and to estimate

climate changes at longer time scales, especially those related to the anthropogenic modification of

the atmospheric composition. Outputs from such models cannot, however, be directly used in

many impact oriented applications because of their relatively coarse spatial scale (typically several

hundreds of kilometers) and associated uncertainties. Furthermore, while particular coarse-

resolution CGCM may be capable of capturing the mean climate behavior, they are not usually

successful in reproducing higher-order statistics and extreme values. Regional climate modelling

plays thus an important role to fill the gap between the global climate models and the growing

demand of climate predictions and scenarios on shorter spatial-temporal scales.

The most important regional climate forcing in the Mediterranean region is associated with the

complex orography characterized in many coastal regions by step mountain slopes and the large

land-sea contrast. This provides a very good testbed but also a big challenge for regional climate

modelling. Determination of the Mediterranean regional climate is currently undertaken through

several different approaches. The most popular one is the use of (usually atmospheric only)

regional climate models (RCM) or limited area models (LAM) (Giorgi and Mearns 1999). Spatial

resolution of such models varies from a few kilometers to several tenths of kilometers, upon the

assumption or not of the hydrostatic approximation. Regional climate models (for example, those

used in Christensen et al. 1997; Jones et al. 1995; Giorgi and Mearns 1999, and many others)

need to be nested into coarser-resolution global models in order to get the necessary driving

information through the frontiers of the domain. This approach allows implementation of much-

detailed physical parameterizations in RCM to ensure a better simulation of local weather and

climate events. Another existing approach is based on the use of variable grid (zoomed) general

91

circulation models (GCM) with closer resolution for the Mediterranean basin (for example, Déqué

and Piedelievre 1995, Li and Conil 2003). This ensures a best downscaling of information from

large scales to regional scale, but the resolution limit is currently thought to be around the 50

kilometers, due to either computing capacity or physical parameterizations implemented in such

GCMs. Third approach for high-resolution determination of climate parameters over the

Mediterranean region is associated with the application of statistical methods for the downscaling

of results issued from climate modelling in large-scale GCMs. Combination of dynamical and

statistical approaches are also possible and have been reported (Krichak et al. 2000).

6.2 Current Status

Numerical modelling, both global and regional, is an important tool to understand physical

mechanisms controlling climate change and variability at different spatial-temporal scales. It also

provides the unique possibility to construct physically-based and comprehensive future climate

scenarios, the starting point for many socio-economical impact considerations. Few studies

dedicated to the Mediterranean regional climate modelling are reported as so far. Most of the

exist ing research programmes, such as Mice, Prudence and Stardex

(http://www.cru.uea.ac.uk/projects/mps), on climate variability and change over Europe include

only partially the Mediterranean basin as the southmost part of their considered domain. Due to the

marginal effects, simulated climates over the Mediterranean basin are often biased by the

prescription of the boundary conditions. This decreases largely the validity of such studies on the

Mediterranean climate. The bias of present climate simulations in the Mediterranean basin is in the

range from -1K to 5K for temperature and from -30% to 20% for precipitation, according a study

reported by Giorgi and Francisco (2000 a,b) with different GCMs.

Regional climate changes under global warming context (Jones et al. 1997, Machenhauer et al.

1998, Frei et al. 2002, Gibelin and Déqué 2003) are the most important motivations for the

Mediterranean regional climate modelling. It is generally agreed that the Mediterranean region is

one of the most sensitive areas on Earth in the context of global climate change, due to its position

at the border of the climatologically determined Hadley cell and the consequent transition character

between two very different climate regimes in the North and in the South.

Many global and regional models tend to simulate a warming of several degrees (from 3 to 7 K) on

the Mediterranean for the end of the 21st century and the warming in Summer is larger than the

global average. There is also a general trend of mean precipitation decrease for the region

(especially in Summer), due to mainly the northward extension of the descending branch of the

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subtropical Hadley circulation. Even when a decrease of total mean precipitation is simulated, the

precipitation amounts in the intensive events may increase in many places (Christensen and

Christensen, 2003). A plausible explanation of more intense extreme events around the

Mediterranean is that a warmer climate contains more water which serves as an additional

potential for latent heat release during the build-up of low-pressure systems.

Another major anthropogenic modification of the Mediterranean natural environment concerns the

land use of the region. There are indications that the albedo change due to the progressive

deforestation in the Mediterranean region since the Roman classical has significantly altered the

atmospheric circulation over northern Africa and the Mediterranean and consequently, the regional

precipitation patterns (Reale and Shukla, 1998).

Besides the important changes which Mediterranean climate may be on the verge of experiencing,

modelling is also a crucial resource to understanding current climate fluctuations, many of which

still need to be satisfactorily explored. For instance, the decadal changes which have characterized

Mediterranean winter precipitation in the last 50 yrs (Hurrell, 1995; Mariotti et al. 2002) and the

related impacts on the Mediterranean environment. On interannual timescales, the so-called heat

waves, which during summertime have been so heavily affecting the quality of life in the

Mediterranean region (Schar et al. 2004). The increasing evidence that the global-scale

fluctuations in tropical sea surface temperature, mostly associated to ENSO, influence

Mediterranean climate, still remains unexplained (Hoerling and Kumar 2003).

6.3 Outlook and Future Challenges

Three important issues are foreseen for the Mediterranean regional climate modelling in the next

few years.

6.3.1 High-Resolution Mediterranean Climate Modelling Systems

The spatial resolution of future modelling systems will be further increased. It is expected to have

models with resolution around 10 to 20 kilometers in the next few years. Experience with numerical

weather forecasting shows that higher spatial resolution usually leads to better prediction, mainly

due to improvements in the representation of atmospheric instability which is crucially dependent

on the model's spatial resolution. In climate modelling, higher spatial resolution may lead to

improvements in some aspects and degradation in others (Leung et al. 2003, May and Roeckner

2001). Climate is in fact more related to the sources and sinks of energy, moisture and momentum.

93

Mechanisms controlling their budgets and evolution at different spatial-temporal scales are thus

crucial for climate. In general higher spatial resolution models can provide a more comfortable

background to incorporate sophisticated physics and the latter will improve the performance of

regional climate models.

The overall studies reported in the current scientific literature seem to show improved model

performance with higher spatial resolution, especially in reproducing extreme events, such as

strong precipitation episodes and cyclogenesis often related to the specific surface orography. But

there is indeed a need to further evaluate and quantify the impacts of spatial resolution on regional

climate simulation. Even in the most advanced high-resolution regional climate models, it will be

difficult, in some cases, to determine dynamically the hydrological variables, such as run-off.

Application of statistical methods will be always necessary to provide appropriate solutions.

In the next few years, high-resolution Mediterranean climate modelling systems are expected to be

used to produce consistent data for the Mediterranean basin during the last 40 years, which can

not be achieved by global re-analysis done in weather prediction centres (such as NCEP and

ERA40) due to the too coarse spatial resolution and the deficiency in the hydrological cycle. By

performing a special calibration through the regional atmospheric/land-surface climate models

covering a quite large domain around the Mediterranean, it is in principle possible to reduce the

hydrological bias of the re-analysis products. Such simulations of the Mediterranean climate over

the last 40 years will be very useful to study the dynamical and physical processes controlling the

climate in the Mediterranean region. They are also useful for climate trend detection for the last 40

years.

6.3.2 Development of Integrated Regional Modelling Systems

Other components controlling the regional climate will enter interactively into the regional modelling

system. They include, through the most important, the Mediterranean Sea general circulation,

basin-scale hydrology, dynamical surface vegetation, land use, atmospheric chemistry, air pollution

and man-made or desert-originated aerosols, marine and land-surface ecosystems. It is expected

that new climate feedbacks and modes yielded through the complex interaction among different

components of the Mediterranean climate system might be discovered and quantified. Especially

the regional atmosphere and Mediterranean sea coupled models should receive high priority for

their development and utilization in the Mediterranean climate studies. From studies currently

undertaken in several European institutes, the high resolution of atmospheric models is quite

crucial to correctly simulate the convection in the Mediterranean sea.

94

With increasing complexity of numerical modelling systems, validation against appropriate

observational data is becoming an important issue. This will require however a significant

improvement of the currently existing data bases for the region and an increasing capacity to

obtain and analyze new measurements with different geophysical characteristics of the region like

soil moisture, soil types, vegetation coverage, dust sources and transport (Tsidulko et al. 2002,

Alpert et al. 2002, Krichak et al. 2002), etc. The current observational network around the

Mediterranean basin is still scarce and accuracy of measured geophysical parameters in this

region is also significantly lower than that over more developed areas like Europe.

Putting the numerical systems in the configuration of paleoclimate will be an interesting exercise to

test the robustness of the numerical models because it is the only way to test the sensitivity of our

complex models to documented climate changes. Paleoclimate simulations will allow to test the

ability of models to simulate the correct amplitude but also geographical pattern of climate changes

thanks to a large number of dated samples all around the Mediterranean basin.

6.3.3 Multi-Model Ensemble Regional Climate Simulations

It is also necessary to emphasize the perspectives of ensemble approach for future Mediterranean

regional climate modelling activities. This is the only way to assess the uncertainties of numerical

modelling for climate variation and probabilistic estimates for changes at long terms. Any climate

impact considerations should take into account this aspect of probability.

In terms of global mean surface air temperature, it is generally agreed that the changes of the

global temperature will be between 2 to 5 degrees at the end of the present century. In broad

terms, the current thinking attributes about half of this range of uncertainty to uncertainties in the

emission scenarios and half to uncertainties in the construction and use of global climate models.

The use of regional climate models will further increase the uncertainty range. We need thus to use

a hierarchy of global and regional models and to run ensemble simulations. This is just at the limit

of our current computing capacity. A more close cooperation with computer industry is thus

necessary in the future to accomplish this task.

6.4 References

Alpert, P., S.O. Krichak, M. Tsidulko, H. Shafir and J.H. Joseph (2002) A dust prediction system

with TOMS initialization., Mon. Wea. Rev.,130, 2335-2345.

Christensen, J. and Christensen O., 2003: CO2 warming and severe summer precipitation, Nature,

in press.

95

Christensen, J.H., Machenhauer, B., Jones, R.G., Schär, C., Ruti, P.M., Castro, M. and Visconti,

G., 1997: Validation of present-day climate simulations over Europe: LAM simulations with

observed boundary conditions , Climate Dynamics, 13, 489-506.

Déqué, M., and Piedelievre J.P., 1995: High-resolution climate simulation over Europe. Climate

Dyn. 10, 249-266.

Frei, C., J. H. Christensen, M. Déqué, D. Jacob, R. G. Jones, and P. L. Vidale, 2002: Daily

precipitation statistics in regional climate models: evaluation and intercomparison for the

European Alps. J. Geophys. Res 108, D3 4124-4142.

Gibelin, A.L., and Déqué, M., 2003, Anthropogenic climate change over the Mediterranean region

simulated by a global variable resolution model. Climate Dyn. 20, 327-339.

Giorgi, F. and Mearns, L.O., 1999: Introduction to special section: Regional climate modelling

revisited , Journal of Geophysical Research, 104, 6335-6352.

Giorgi F., and Francisco R., 2000a: Evaluating uncertainties in the predictioon of regional climate

change, Geophys. Res. Lett., 27, 1295-1298.

Giorgi F., and Francisco R., 2000b, Uncertainties in regional climate prediction: a regional analysis

of ensemble simulations with the HADCM2 coupled AOGCM, Clim.Dyn., 16, 169-182.

Hoerling M. and Kumar, A., 2003, The perfect ocean for drought, Science, 299, 691-694

Hurrell, J.W., 1995: Decadal trends in the North Atlantic Oscillation: regional temperatures and

precipitation. Science, 269, 676-679.

Jones, R.G., Murphy, J.M., and Noguer M., 1995: Simulation of climate change over Europe using

a nested regional-climate model: I: Assessment of control climate, including sensitivity to

location of lateral boundaries. Quarterly Journal of the Royal Meteorological Society, 121,

1413-1449.

Jones, R.G., Murphy, J.M., Noguer, M. and Keen, A.B., 1997: Simulation of climate change over

Europe using a nested regional-climate model. II: Comparison of driving and regional model

responses to a doubling of carbon dioxide. Quarterly Journal of the Royal Meteorological

Society, 123, 265-292.

Krichak, S.O., M. Tsidulko and P. Alpert (2000) Monthly Synoptic Patterns Associated with Wet/Dry

Eastern Mediterranean Conditions, Theoretical and Applied Climatology, 65, 215-229.

Krichak, S. O., M. Tsidulko, and P. Alpert, A study of an INDOEX period with aerosol transport to

the eastern Mediterranean area,(2002)J. Geophys. Res., 107(D21), 4582,

doi:10.1029/2001JD001169.

Leung, L.R., Means, L.O., Giorgi, F., and Wilby R.L., 2003: Regional climate research, needs and

opportunities. Bull. of the American Met. Soc., Jan. 2003, 89-95.

Li, Z.X., S. Conil, 2003: Transient response of an atmospheric GCM to North Atlantic SST

anomalies, J. Climate, 16, 3993-3998.

96

Machenhauer, B., M. Windelband, M. Botzet, J.H. Christensen, M. Deque R.G. Jones, P.M. Ruti,

and G. Visconti (1998). Validation and analysis of regional present-day climate and climate

change simulations over Europe. MPI Report No.275, MPI, Hamburg, Germany.

Mariotti, A., M.V. Struglia, N. Zeng, K.-M. Lau, 2002: The hydrological cycle in the Mediterranean

region and implications for the water budget of the Mediterranean Sea, J Climate, 15, 1674-

1690.

Mearns, L.O., Bogardi, I., Giorgi, F., Matyasovszky, I. and Palecki, M., 1999: Comparison of

climate change scenarios generated from regional climate model experiments and statistical

downscaling , J. of Geophys. Res., 104, 6603-6621.

Reale O. and J. Shukla, 1998: Modeling the effects of vegetation on Mediterranean climate during

the Roman classical period. Part II, high resolution model simulation. ICTP Preprint

No.IC98096, 31pp, available at http://www.ictp.trieste.it/ictp/preprints

Schar, C., P.L. Vidale, D. Luthi, C. Frei, C. Haberli, M.A. Liniger, C. Appenzeller, 2004: The role of

increasing temperature variability in European summer heatwaves, Nature, 427, 332-336.

Tsidulko, M., S. O. Krichak, P. Alpert, O. Kakaliagou, G. Kallos, and A. Papadopoulos, 2002:

Numerical study of a very intensive eastern Mediterranean dust storm, 13-16 March 1998, J.

Geophys. Res., 107(D21), 4581, doi:10.1029/2001JD001168.

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7. Aspects of Climate Dynamics Due to the Sun

Wilhelm May1, Henrik Feddersen1, Joanna Haigh2, John Harries2, Ralf Toumi2, Wenyi Zhong2, Jón

Egill Kristjánsson3, Jørn Kristiansen3, Trude Storelvmo3, Peter Thejll1, Hans Gleisner1, Bo

Christiansen1, Eigil Kaas1 and Martin Stende12

1 Danish Meteorological Institute, Denmark ([email protected], [email protected], [email protected], [email protected],[email protected], [email protected], [email protected])

2 Imperial College of Science and Technology, UK ([email protected], [email protected],[email protected])

3 University of Oslo, Norway ([email protected], [email protected], [email protected])

Climate variability is due to several causes – internal atmospheric dynamics, variations in external

forcing and the ocean-atmosphere coupling. To understand the relative importance of these factors

we need to understand the role played by each. Some studies based on observational data,

suggest that the upper tropical troposphere is warmer and moister at solar maximum than at solar

minimum. Other studies indicate a broadening and weakening of the Hadley cell at solar maximum

resulting in a poleward displacement of the mid-latitude storm tracks. The MED region might be

particularly vulnerable to this shift due to its dependence on extratropical cyclone activity in winter

for water supply.

The mechanisms that link solar variability to climate processes should be studied in order to

explain these observations. The study should include both data-analysis building on recent

advances in detecting and characterising solar signals (Gleisner & Thejll 2003, Thejll et al. 2003),

and model-studies focusing on amplifying processes, for instance the role played by ozone (Haigh

1994), vertical dynamical coupling [Christiansen, 2001], clouds and the hydrological cycle

(Kristjánsson et al. 2003). Simplified GCM models should be used as diagnostic tools to

understand feed-back mechanisms during radiative heating (Haigh 1996, 1999, 2001, 2003,

Labitzke et al. 2002) in particular with respect to the reported solar signal in low clouds

(Kristjansson et al. 2002) and SSTs (White et al. 1997).

7.1 References

Christiansen, B. 2001. Downward propagation of zonal mean zonal wind anomalies from the

stratosphere to the troposphere: Model and reanalysis, J. Geophys. Res., 106, 27307-27322,

2001.

Gleisner, H. and P. Thejll, 2003. Patterns of tropospheric response to solar variability. Geop. Res.

Lett., 30, 711, 2003

Haigh, J.D. 1994. The role of stratospheric ozone in modulating the solar radiative forcing of

climate. Nature, 370, 544-546.

Haigh, J.D. 1996. The impact of solar variability on climate. Science, 272, 981-984.

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Haigh, J.D. 1999. A GCM study of climate change in response to the 11-year solar cycle.

QJRMS., 125, 871-892.

Haigh, J.D. 2001. Climate variability and the role of the Sun. Science, 294, 2109-2111.

Haigh, J.D. 2003. The effects of solar variability on the Earth’s climate. Phil.Trans.Roy.Soc A., 361,

95-111.

Kristjánsson, J. E., A. Staple, J. Kristiansen, and E. Kaas, 2002. A new look at possible

connections between solar activity, clouds and climate. Geophys. Res. Lett., 29, 23, 2107,

10.1029/2002GL015646.

Kristjánsson, J. E., J. Kristiansen, and E. Kaas, 2003. Solar activity, cosmic rays, clouds and

climate - an update. Adv. Space Res. (in press).

Labitzke K, J Austin, N Butchart, J Knight, M Takahashi, M Nakamoto, T Nagashima, J Haigh and

V Williams, 2002. The global signal of the 11-year solar cycle in the stratosphere:

Observations and models, JASTP, 64, 203-210.

Thejll, P., B. Christiansen and H. Gleisner, 2003. On correlations between the North Atlantic

Oscillation, geopotential heights, and geomagnetic activity. Geop. Res. Lett., 30, 6, 1347,

2003

White, W.B., J. Lean, D.R. Cayan and M.D. Dettinger, 1997. A response of global upper ocean

temperature to changing solar irradiance. J. Geophys. Res., 102, 3255-3266.

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